Split capital cost in FOM + investment cost

Instead of combining fixed operationand maintenance (FOM) plus annualised investment costs in one attribute, currently called capital_cost, split them into 2 attributes and change the objective function accordingly.

This would make it possible to
(i) store information on FOM and annualised investment costs directly in the pypsa network and
(ii) also to better calculate the costs of, for example, an early shutdown of a power plant before the end of its lifetime. In the event of an early shutdown, the annualised investment costs would still have to be paid, but not the FOM.

Suggested steps would be:

• replace attribute capital_cost for all components by two new attributes investment_cost and FOM
• change objective function to consider investment_cost from all build assets and FOM from all active assets

Another option that breaks less with the existing implementation of the costs would be to add only the FOM and keep the current definition of capital_cost. In case of early phase-out of a power plant, the FOM could be subtracted from the capital_cost to only consider the annualised investment costs.

@coroa did you include this already into pypsa?