Improve how the offshore NEP is forced in

Copying from PyPSA/pypsa-ariadne#191

Open points:

• The way the land use constrained is hacked seems a bit strange. I am worried that the p_nom_max is now too high, because the values are just copied over from the north_sea_duck_node generator. It's no problem as long as the capacity is limited by a global constraint. But should be rewritten later on.
• Implementation of AC rests on the assumption that the connection point already exists from previous years. might potentially error if later projects connect farther inland. Probably fine
• Ideally we would have separate, non extendable generators, with "NEP" in their name. Those would have the correct capacities and are not extendable. We could even delete all extendable offwind generators <2030 and only force in capacities in add_existing and in the NEP_force_function. As a caveat, this might throw off n.statistics. Potentiall the new active option in PyPSA might help here.

Start by tracing the NEP_force_function and add_existing entry points, then inspect how capacities are copied from the north_sea_duck_node generator. Resolve the listed questions about land-use limits, AC connection points, non-extendable NEP generators, and n.statistics before considering the work done.