287 feature extract multiple columns from one prejoined object

[tkiehn]

Add new syntax for prejoins, allowing extraction of multiple columns from the same prejoin-source:

  prejoined_columns:
      - extract_columns: 
           - id
           - number
        aliases:
           - businessid
           - businessnumber
        ref_model: 'business_raw'
        this_column_name: 'ContractId'
        ref_column_name: 'ContractId'

In this example we join the source model with business_raw and extract id and number and assign an alias for each column.
Assigning an alias is optional, but if done, the number of extract_columns has to match the amount of aliases.
For each prejoin-target a new list, containing at least a dictionary for extract_columns and values for ref_model(or source_name & source_table) and the column names, needs to be defined.

aliases are optional, but if given, must be the same length as extract_columns.
this_column_name & ref_column_name allow multiple columns
ref_model are changeable to src_name & src_table

The old syntax is still valid and can be used if desired.

Fixes #287

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation or included information that needs updates

ToDO: Update Docs to include the new syntax

[tkiehn]

Merged into test (0557fbf)

[tkiehn]

Closed in favor of #300