Add `capacity_in_mw` to flex model for scheduling

[nhoening]

We need to give capacity constraints a more prominent place in scheduling. That place is flex_model. At the same time, the capacity_in_mw attribute on sensors and/or assets needs to become an optional fallback.

Adding it to flex_model

I suggest we keep this issue overseeable by allowing a fixed value. Issue #761 has been made to accommodate to represent the capacity of a sensor by another sensor, which allows for more dynamic grid service settings.

The attribute on sensor or asset should still be a possible fallback.

This also needs documentation, which should be in the new section talking about flexibility notation (for now, still in the API section).

Making capacity_in_mw optional

The capacity_in_mw attribute is required in some parts of the code. This ticket will replace any logic using it, such that it becomes a completely optional attribute.

Next to the scheduling case discussed above, it's at least also used to set the scale range on the latest state plot. Here, we can use a max on the history, or let it go altogether (let the chart determine the scale range) - I'd prefer the latter approach.

Originally posted by @Flix6x in #334 (review)

[Flix6x]

Update: By now, the latest state plot has already been deprecated.

[victorgarcia98]

For this, we'll need to create 2 fields to expose the (power) capacity of the storage and the the EMS, separately. I suggest using the following field names:

• "storage-max-power" or "storage-power-capacity"
• "ems-max-power", alternatively, "ems-power-capacity"

Note: this could be asymmetric but let's start with supporting symmetric capacity constraints trough the scheduler.