Fix: Storage Investment

[jnnr]

Storage investment with existing capacities leads to unexpected results, see rl-institut/oemoflex#42 (comment).

This PR

• Introduces constraint tests for tabular facades,
• fixes the described bug by passing the existing capacity to the investment object of the output flow,
• calculates the maximum additional investment as the potential minus existing capacity/storage capacity.

TODO:

• Resulting LP file for brown field is wrong again after updating to solph v0.4. The expected files have been checked thouroughly before and should be correct. update: locally, the constraint tests pass now
• In facades, l. 122: Shouldn't the potential be set even when no costs are given? Or, alternatively, an Exception be raised? Will be set in any case. Done in 59aff4f
• Check if new understanding of capacity_potential is consistent accross all facades
• Write some lines to whatsnew.

[henhuy]

Hey @jnnr,
looks good to me.
I refactored _investment function a bit - so that it is not too nested!

pytest fails - refactoring seemed to change something. I will check