Fix/battery in out flow

[j-ti]

branched from #113

initially developed for model predictive control update to fix #218

[j-brendel]

As I understand it, the problem prevails with this fix:

When setting "incorrect" variable costs, the battery charges and is beeing charged in the same timestep

[j-ti]

As I understand it, the problem prevails with this fix:
When setting "incorrect" variable costs, the battery charges and is beeing charged in the same timestep

I constructed it in the way that you have to add a parameter to the model description in order to use it
'constraint_flow_keyword': 'battery_flow',

each component needs an individual keyword such as "battery_flow" in order to identify the group of flows of the model which should just have one active flow.

Currently, I have a fixed list ['battery_flow'] for test purposes but it there is a todo in run_smooth:
# TODO: Extract keyword list of constraint flows from model description keywords = ["battery_flow"]

[j-brendel]

I made a new commit, where I moved the keyword to the general component.py initialization as I think this could be useful for all components.
Here is an example: I used 2 Batteries, in the left picture the constraint_flow_keyword ist equal for both, in the right they differ. I think this makes the use well visible.