Use pint to check and convert units

[Flix6x]

Goal is to support other (power) units throughout FlexMeasures, e.g. by converting incoming data to the sensor unit, and outgoing data to the requested unit.

The pint-pandas package doesn't work too nicely with all of the pandas operations we use in timely-beliefs, so instead I propose to use pint on Sensor.unit, instead of on BeliefsDataFrame["event_values"]. I'd also like to keep the Sensor.unit database column as is, i.e. storing the shorthand string representation of units (e.g. MW, m/s and EUR/kWh).

I've identified some issues that would need to be resolved in order for this to work smoothly, solutions for which are all within reach:

• We use "h" to denote hour, whereas pint uses "h" to denote Planck's constant (and "hr" for hour). "h" should be the abbreviation of hours hgrecco/pint#719 (comment)
• Currencies are not included by default. https://pint.readthedocs.io/en/stable/currencies.html?highlight=currency#full-model
• Wh/h is not automatically simplified to kW. Feature request: What kind of quantity a unit represents hgrecco/pint#676 (comment)