Add docs

[jnnr]

This PR adds a first version of documentation to the repo.

Build locally with sphinx-build docs <path-to-build> or have a look at the online version built by rtd:

https://oemoflex.readthedocs.io/en/features-docs/

Compare to https://oemof-b3.readthedocs.io/en/latest/model_structure.html

TODO:

• [x] include components.csv and component_attrs in docs
• [x] Missing input data has to be provided in the documentation
• [x] Results template has to be provided
• [x] CBC solver has to be installed
• [x] oemof.tabular dev branch must be used.
• [x] Quick note about the csv field separator when using input_parameters (#57)

[unndreay]

We have 3 different short descriptions now:

Oldest: https://github.com/modex-flexmex/oemo-flex/blob/18c163741e0b4cbf728dc10141897b229c2fe489/docs/getting_started.rst#L7

Later, required because 'overview': https://github.com/modex-flexmex/oemo-flex/blob/18c163741e0b4cbf728dc10141897b229c2fe489/docs/overview.rst#L7

Identical with the one in getting_started.rst, but changed recently: https://github.com/modex-flexmex/oemo-flex/blob/18c163741e0b4cbf728dc10141897b229c2fe489/docs/model_structure.rst#L7

Which one to keep/to drop?

[jnnr]

We have 3 different short descriptions now: Which one to keep/to drop?

Proposal:

• Drop the one in getting started.
• Move the one from model structure to overview. Put the existing sentence in overview in a second sentence, in the lines of "in the background, it uses oemof.solph and oemof.tabular"

[unndreay]

Travis fails. There are new flake8 messages after ignoring E265 has been introduced: https://github.com/modex-flexmex/oemo-flex/blob/019d8f4dc45261d9877a81814ad7fb87251ce9c1/.flake8#L3

Cannot find the real source of that. A flake8 bug? Workaround: per-file-ignore (5e16e67).

EDIT: Not a bug. 'ignore' overwrites the default ignore list!