Return dynamics's generators from Model

[frosati1]

What

Ideally reinserts lindbladian evolution

Why

Linbladian evolution was in the code but not accessible.
Reduces code complexity and nested ifs.
Closes Issue #215

How

A new function in the Model takes care of returning Hamiltonians or Liouvillian superoperators according to model.lindbladian.
In this way propagation.pwc() basically just needs to do the exponentiation.

Remarks

Checks on signals (e. g. passing times) are supposed to be moved to the Generator and might be faulty now.

Checklist

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• Tests - Added unit tests for new code, regression tests for bugs and updated the integration tests if required
• Formatting & Linting - black and flake8 have been used to ensure styling guidelines are met
• Type Annotations - All new code has been type annotated in the function signatures using type hints
• Docstrings - Docstrings have been provided for functions in the numpydoc style
• Documentation - The tutorial style documentation has been updated to explain changes & new features
• Notebooks - Example notebooks have been updated to incorporate changes and new features
• Changelog - A short note on this PR has been added to the Upcoming Release section

[frosati1]

Goes on with @SRaent PR, I delete this