Subclass for Emission class

[redfrexx]

So far we have one class Emissions that contains all meta information on the calculated emissions. It is used for trips only so far. Once we implement the new API for buildings emissions as well we need to add subclasses for the different emission types, because some attributes are only relevant for one of them, e.g. distance is only relevant for trips.

Example: TripEmissions and EnergyEmissions