This implementation works as in generating an lp-file.
Different working state and LP file need further attention.

If bev is True, all investment parameters are given
but no ep_costs. This is because multiple components
are linked within one bev facade. But only the in flow
of the storage shall have the aggregated costs for the
whole facade.

…evel

The equal-invest constraint is not necessary anymore as
the flow through the bev-facade is already restricted by the
storage in & outflow. Costs are only applied for the storage.

Although not necessary, we constraint will still be applied to check
the lp files.
This reverts commit 7472a15.

This test is not necessary as there are no further features
compared to the single period one.

Tests for BEV facades