Introduce unit scale for better managing energy (kW) vs. flow units (m3)

Fix #121 and #196.

**NOTE:** users will need to update their frontend settings for and `cost`, `initialconsumption` if they use non-energy meters.

Please have a careful look and test. I'd especially appreciate the following tests:

  - el. meter
  - gas meter (@mbehr1)
  - water meter (@mbehr1)
  - el. meter with SensorInterpreter
  - gas meter with SensorInterpreter (anyone?)