Fix sphinx warnings when building docs

[meliache]

When building the sphinx documentation (via running make html in the docs dir, using sphinx 3.0.2) I get the warnings seen below. I think it should be not much work to fix them.
Further, it would be good to have some automated CI-procedure (test, hook or whatever) to do the build and disallow commits/PR's which introduce warnings.

/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/core/task.py:docstring of b2luigi.Task.get_input_file_names_from_dict:13: WARNING: Definition list ends without a blank line; unexpected unindent.
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/core/task.py:docstring of b2luigi.Task.get_input_file_names_from_dict:18: WARNING: Definition list ends without a blank line; unexpected unindent.
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/core/task.py:docstring of b2luigi.Task.get_input_file_names_from_dict:20: WARNING: Unexpected indentation.
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/core/task.py:docstring of b2luigi.Task.get_input_file_names_from_dict:21: WARNING: Block quote ends without a blank line; unexpected unindent.
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/core/task.py:docstring of b2luigi.Task.get_input_file_names_from_dict:27: WARNING: Definition list ends without a blank line; unexpected unindent.
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/batch/processes/gbasf2.py:docstring of b2luigi.batch.processes.gbasf2.Gbasf2Process:53: WARNING: Unexpected indentation.

/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/data.py:docstring of b2luigi.basf2_helper.data.CdstDataTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/data.py:docstring of b2luigi.basf2_helper.data.DstDataTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/data.py:docstring of b2luigi.basf2_helper.data.MdstDataTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/data.py:docstring of b2luigi.basf2_helper.data.RawDataTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/data.py:docstring of b2luigi.basf2_helper.data.SkimmedRawDataTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/tasks.py:docstring of b2luigi.basf2_helper.tasks.MergerTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/tasks.py:docstring of b2luigi.basf2_helper.tasks.SimplifiedOutputBasf2Task.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)

[meliache]

The code formatting here is just broken: https://b2luigi.readthedocs.io/en/stable/documentation/api.html#b2luigi.Task.get_input_file_names_from_dict

Maybe @welschma can take a look when he has some time?

[meliache]

I fixed the first half of warnings including the broken formatting in #66. The second half of warnings still remains:

/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/data.py:docstring of b2luigi.basf2_helper.data.CdstDataTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/data.py:docstring of b2luigi.basf2_helper.data.DstDataTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/data.py:docstring of b2luigi.basf2_helper.data.MdstDataTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/data.py:docstring of b2luigi.basf2_helper.data.RawDataTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/data.py:docstring of b2luigi.basf2_helper.data.SkimmedRawDataTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/tasks.py:docstring of b2luigi.basf2_helper.tasks.MergerTask.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)
/home/michael/basf2/externals/v01-09-01/Linux_x86_64/common/lib/python3.6/site-packages/b2luigi/basf2_helper/tasks.py:docstring of b2luigi.basf2_helper.tasks.SimplifiedOutputBasf2Task.output:13: WARNING: undefined label: task.output (if the link has no caption the label must precede a section header)