Let `flex_model.capacity_in_mw` be a sensor

[nhoening]

#335 adds it as a fixed value, which is fine for many assets.

On a system level with grid services, we will encounter changing maximal capacities, so a sensor ID would be very useful.

• A new field capacity-sensor would be added to the flex_model, parsed and documented
• We need to either use that or the attribute from Add capacity_in_mw to flex model for scheduling #335, not both
• The schedulers need to be able to use a series, not a fixed value