feature: flex context prices as quantities

[Flix6x]

Description

QuantityOrSensor("/MWh") can now be used for energy price fields. Any unit is accepted as a numerator: this includes currencies, such as EUR/MWh, USD/MWh and AUD/MWh, but also kg/MWh in case of scheduling against emissions, or even just %, which would get converted to units of MWh/MWh.

How to test

I expanded a test for the schema and added an extra test case.

Related Items

Closes #1027.