Support saving state-of-charge schedules to sensors with `"%"` unit

[Flix6x]

In flexmeasures/data/models/planning/storage.py we have the below bit of code that converts the state-of-charge schedule to the unit of the sensor where the schedule needs to be recorded. The convert_units method accepts a capacity argument with which the MWh values can be converted into % values. The capacity needs to be read from the flex-model for the given device, specifically, from the soc-max field.

• If the field contains a constant unit (e.g. "60 kWh") than use that.
• Note that the soc-max field is a QuantityField, so it cannot contain a sensor reference (e.g. {"sensor": 50})
• If the field is missing or zero, the conversion cannot happen. However, do not crash the scheduler, just don't save the state-of-charge schedule.
• Add test coverage
• Update the STATE_OF_CHARGE field metadata documented in flexmeasures/data/schemas/scheduling/metadata.py
• Run all precommit hooks before committing
• Make sure all tests pass

        soc_schedule = {
            flex_model_d["state_of_charge"]: convert_units(
                integrate_time_series(
                    series=ems_schedule[d],
                    initial_stock=soc_at_start[d],
                    stock_delta=device_constraints[d]["stock delta"]
                    * resolution
                    / timedelta(hours=1),
                    up_efficiency=device_constraints[d]["derivative up efficiency"],
                    down_efficiency=device_constraints[d]["derivative down efficiency"],
                    storage_efficiency=device_constraints[d]["efficiency"]
                    .astype(float)
                    .fillna(1),
                ),
                from_unit="MWh",
                to_unit=flex_model_d["state_of_charge"].unit,
            )
            for d, flex_model_d in enumerate(flex_model)
            if isinstance(flex_model_d.get("state_of_charge", None), Sensor)
        }