SMETS2 gas meter consumption is measured in m3 rather than kWh, so conversion
from one unit to the other is required.
Algorithm and description of terms is taken from:
https://www.theenergyshop.com/guides/how-to-convert-gas-units-to-kwh
This commit adds additional config file options to the `gas` section:
- `meter_type` (default `1` for backward compatibility)
Enables unit conversion from m3 to kWh before reporting metrics to influxdb
- `volume_correction_factor` (default `1.02264`)
- `calorific_value` (default `40`)
