Refactor measurement endpoints to be generic

[stormaaja]

Remove dependency to temperature. Instead take measurement type and field from the path.

Add validation though. Allow only

• temperature and temperature
  • /measurements/temperature/{sensorId}/temperature
• electricity_consumption and energy
  • /measurements/electricity_consumption/{relayId}/energy

Use grouping: https://gin-gonic.com/docs/examples/grouping-routes/

Rename store route to measurement route.

Rleated to #7