Enhance documentation for reactive power calculation

Currently, the data model leaves open to some extend, how capacitive and inductive reactive power of assets can be defined. However, introducing just a flag to toggle the behavior is not sufficient, because then all reactive power characteristics would have to provide the same information as well.

Proposal from my side:

• The sign of the power factor defines the sign of reactive power: Positive means reactive power is consumed, regardless of if active power is consumed or not
• For assets that change between infeed and consumption and if a active power dependent factor shall be used, then the characteristic needs to be defined for negative values as well, if also the reactive power direction is meant to change

It would have the advantage, that it would also work with the definition of characteristics.