Rename "nominal_value" of Flow

[p-snft]

For Flow, we have the keyword nominal_value. 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.

[upadhayaajay]

I occasionally find it confusing; using the name nominal_value seems less intuitive. It might make more sense to name it nominal_capacity as you suggested, providing a clearer indication of its meaning. I agree with your suggestion to rename both to nominal_capacity.

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[p-snft]

I still have to keep explaining people what the nominal_value is meant for and they just understand if I name it nominal_capacity, instead. I think, we should really do this.

[lensum]

I agree that nominal_capacity would be an easier-to-understand term for the nominal_value in the flow. In fact, we already use this term in the documentation of the full_load_time_max and full_load_time_min. However, I also think that it could be confusing to have nominal_capacity describe (simplified) the power of a flow, while the nominal_storage_capacity in the GenericStorage is an energetic quantity and not a power quantity.
Would it be enough to include good documentation on the nominal_storage_capacity? Because I can't think of a better term than "capacity" to describe both quantities (see also this Wikipedia Article that lists terminology, but most of them end in "capacity"). I guess users that don't read the documentation carefully could be confused. Though maybe this is something we have to live with?

[p-snft]

I guess that users that do not consciously distinct power and energy will stumble upon the different time index at some point. In my opinion, the new introductions (#1141) should be very distinct here. Also, the new result processing (cf. #1138) might make the difference clearer by not using these different types of data with the same index (that they in fact do not share).