Implement mileage constraint

[nailend]

The mileage: constraints the total sum of driven km over all timesteps for each vehicletype e.g BEV-Fleet(v2g, g2v, inflex together), or ICE-diesel

For this, the sum of the flow connected to the demand-bus is taken over all timesteps in a year and set equal to this value. This needs to be implemented for all vehicle-types. This is similar to the GeneralIntegralLimit-constraint which is used for emissions.