Feature/#3 multiple regions from csv

Fixes #3

Now it is possible to run simBEV for multiple regions. In short:

added dir scenarios with 2 examples (default_single + default_multi) (howto see scenarios/README.md)
command line arg is now scenario (default: default_single)
scenario type is defined in the header of simbev_config.cfg:
- if multi is used, scenario data is read from CSV files
- if single is used the defined params from config are converted into DFs which have the same structure as the CSVs used in multi scenarios

One reviewer should be enough.

Please review this before #22 !

rl-institut / simbev