(Re)move postprocessing

[nesnoj]

Description of the issue

There's a lot of postprocessing material which is not part of the core functions of open-mastr. Also, much of the code likely won't work no more.

Related: #411

Ideas of solution

Move to separate postprocessing repo.

Another idea: Use nbsphinx to render jupy nbs in docs
Example for rendered NBs: https://windnode-abw.readthedocs.io/en/latest/notebooks/scenario_analysis_NEP_RE-.html