Expose total consumption in frontend

Added `intitialconsumption` optional property. 

If `intitialconsumption` is set to a numeric value, frontend will display total consumption as

    1000 * intitialconsumption + consumption since t=0

Thus, `initialconsumption` can be used to define the meter consumption at the starting time of monitoring. Frontend is updated asychronously as the function could potentially be expensive on the database, especially if aggregation is not used.

For `SensorInterpreters` the restrictions of https://github.com/volkszaehler/volkszaehler.org/issues/158 apply.