trigger_and_get_schedule and get_schedule methods should accept unit

[Flix6x]

Let's keep it simple, as we only need support for a few cases:

def convert_units(values: list[int | float], from_unit: str, to_unit: str) -> dict:
    """Convert values between W, kW and MW, as required."
    if from_unit == "MW" and to_unit == "W":
        values = [v * 10**6 for v in values]
    elif (from_unit == "MW" and to_unit == "kW) or (from_unit == "kW" and to_unit == "W"):
        values = [v * 10**3 for v in values]
    elif from_unit == to_unit:
        pass
    elif (from_unit == "W" and to_unit == "kW) or (from_unit == "kW" and to_unit == "MW"):
        values = [v * 10**-3 for v in values]
    elif from_unit == "W" and to_unit == "MW":
        values = [v * 10**-6 for v in values]
    else:
         raise NotImplementedError(f"Power conversion from {from_unit} to {to_unit} is not supported.")
    return values

async def get_schedule(
    ...
    unit: str,  # <----------
) -> dict:
    ...
    check_for_status(status, 200)

    # Convert to the requested unit
    from_unit = schedule["unit"]  # from the schedule
    to_unit = unit  # from the function argument
    schedule["values"] = convert_units(schedule["values"], from_unit, to_unit)

    return schedule

Potential follow-up: open a FlexMeasures issue to handle this server-side. Then, after the feature lands and the host server has been upgraded, it should be possible to simply pass the desired unit in the [GET] schedule request.