DER state space model automatic scaling / normalization

[stroitzsch]

Implementation of automatic scaling / normalization of state space matrices. Thus, translation state space variables into appropriate per-unit values to avoid numerical issues during optimization in CPLEX / Gurobi.

• [x] Formulate scaling approach (how to define normalization base, etc.).
• [x] Add generalized scaling method to FlexibleDERModel.
• [x] Consider scaling when defining objective values.
• [x] Consider scaling when obtaining simulation / optimization results.
• [x] Validate behavior for all DER model types.

[stroitzsch]

Finalized as of 8bffac391c3f52f49bfabe80034e9e7a9abd36bd