Take resolution into account for maximum size of planning window

[nhoening]

The planning window (currently FLEXMEASURES_MAX_PLANNING_HORIZON) should be based on the sensor resolution, times a factor.

The number of steps for planning massively affects CPU time for computing a schedule.

We have cases where updates happen every 5 minutes, and it's okay to plan 24 hours ahead (12 * 24 = 288 steps)
Other cases have an hourly resolution, but want a week ahead (24 * 7 = 168 steps).

[Flix6x]

I suggest to repurpose FLEXMEASURES_MAX_PLANNING_HORIZON for this rather than introduce a new config setting. Based on its type, we'd have these options:

• None: no maximum is enforced
• int: max number of steps of the sensor's resolution
• timedelta: max duration