Add functionality of optimal state of charge

[PStange]

Add desired SoC for stand-by periods of storages. If not implemented, the solver tend to minimize the SoC for reducing losses. This might not be favourable in terms of supply security or regarding lifetime of batteries.

[FBumann]

If the "stand by periods" are predefined periods, this can be achieved by using min_rel_charge_state
If not, can you make a description, how such a stand by period can be described?
@PStange