Rework labels and colors

[jnnr]

This PR simplifies the usage of labels and colors by

• listing simple labels instead of tuple labels in the labels dictionary

TODO:
Test compatibility with

• examples
• oemof-B3 examples Use model-own labels and colors for plotting oemof-B3#64
• oemof-B3 toy scenario Set up toy scenarios oemof-B3#43
• oemof-B3 plot resources Features/plot scalar resources oemof-B3#50

[juliusmeier]

Nicely simplified.
Currently, rl-institut/oemof-B3#50 (Features/plot scalar resources) does not need the map labels function because only the conventional power plant data is plotted which has its own labels style originating from OPSD. Therefore an individual labels dict is created in the script to rename the labels. The colors work, though.