Revision/rename nominal value

[p-snft]

I think, the capacity of a Flow (e.g. a power line) is actually easier to interpret as the "nominal value". Actually, I have been asked that quite some times. If we have a nominal_capacity of a Flow, it would be arguable why it's nominal_storage_capacity for the GenericStorage. So we need to say either nominal_flow_capacity which is actually redundant. In my opinion, it makes sense to call both nominal_capacity.

In fact, they are quite similar. In particular, they both accept a number (fixed capacity) or an Investment object.