Add test cases for storage investment - #42

Added constraint tests for storage and asymmetric storage and checked the lp-files:

• storage_investment_greenfield: Ok
• asymmetric_storage_investment_greenfield: Ok
• storage_investment_brown_field:
  • Bug in constraint c_e_GenericInvestmentStorageBlock_power_coupled(storage)_
    Input and output capacity should be the same.
    Fix: Pass self.existing to the invest object of the output flow.
  • Not a bug, but may be better to adapt it: Currently, potentials relate to the invested capacity on top of existing, not on the sum of existing and additionally invested. E.g., if there is an existing capacity of 3 (MW) and a potential of 5 (MW), the max would be 8 MW. Usually though, potential means the total potential, including the part of it that is already realized. Should pass max = potential - existing to the investment objects.
  • Both are addressed in Fix: Storage Investment oemof/oemof-tabular#33
• asymmetric_storage_investment_brown_field:
  • Again, potentials relate to invested capacity, irrespective of how much capacity already exists. This does not reflect the common usage of "potential". Defined in the bounds of the investment variables.
  • Done in dc67789