Refactor ThermalGridDemand

[danielfeismann]

In my opinion the ThermalGridDemand should

• also be able to use for negative values to show some 'production'

Definitions in ScalaDoc state required as the absolutely required demand and possible for the maximum demand. We named it additionalDemand later, which is a bit misleading. In my point of view we should use it in the meaning of min and max. Thus

• def hasAdditionalDemand should check if possible > zero not > required
• Optional: Maybe some functions for hasRequiredNegativeDemand (maybe find a better wording) make sense?
• ... more ... ?

Not sure about:

• We should throw an exception if possible < required or at least think if the actual solution to set ThermalDemand to (possible, possible) always make sense and is what we expect..

I started but maybe someone would take over this