Unit validation on price sensors in optimization context

[Flix6x]

From #451 (comment):

Our scheduled sensors typically record flows, and our most typical case is a kW power sensor priced by a EUR/kWh price sensor. (We first compute the change in kWh stock by multiplying the power sensor values by the resolution.) I believe it should be valid to swap out EUR for anything else worth minimizing, and I'd also want to support any combination of metric prefixes (W, kW, MW).

In general, I'd start with enforcing the assumption that the denominator of the price sensor is <unit of time> times the unit of the power sensor. And I'd allow anything as the numerator of the price sensor.