Add tables to docstrings of plots and test plot functions

[jnnr]

This adds tables to the docstrings of plot functions which show what data format the plot expects.
The tables are created from csv files that lie in tests/_files/plot_data. This allows us to use the same data to write tests (can happen later or in this PR).

This will help to better understand and maintain the plotting functions collected here. More of oemof-B3s plotting can relocate here and be re-used (and ultimately send to oemof.tabular).

[jnnr]

@monika-o or @Stefanie08, can one of you take over this PR and do the following for each of the function which has been given a table:

• Find out how the DataFrame which the function expects looks like.
• Create a small example in tests/_files/plot_data and name it input_<name-of-the-function>
• Update the link in the docstring of the function to point to the file you created.

It may be that most of the functions expect data of the same form. In that case, do not create a csv for each function, but only one. You can choose a more general name for the csv in that case.