Battery electric vehicle meta facade

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Description

This is some extensive development for BEV component. This is to be merged in #94 and then merged into dev.

The meta-facade incorporates all three bev-techs (v2g, g2v, inflex).
There is an summary node connecting all techs to the electricity bus and one connecting all techs to the demand bus.
This as especially helpful for implementing a mileage constraint (absolute sum of km driven by all bevtechs together in one year). This way only the sum over all timesteps for the flow between "BEV-total" and "pkm-bus" needs to be equal to this amount.

The following graph shows a small energy system graph with the BEV-fleet meta facade and was created in the small example case in https://github.com/oemof/oemof-tabular/blob/feature/battery-electric-vehicle_meta-facade/tests/mobility.py

• squares are the storage
• diamonds are conversions
• circles are buses/nodes besides wind/excess/shortage/load
• pkm_demand is a sink

Flex share: is the share between the different bev-techs. The constraint already exists see here

What is still missing:

• market-shares Implement transport market share constraint #167
• mileage constraint Implement mileage constraint #169

Type of change

Please tick or delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

Please tick or delete options that are not relevant.

• New and adjusted code is formatted using the pre-commit hooks
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• If new packages are needed, I added them the setup.py
• I have added new features/fixes to the CHANGELOG
• I have added my name to AUTHORS