Handle unit conversion for SMETS2 gas meters

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`)

example-octograph.ini

@@ -25,3 +25,9 @@ mpan = 12345
 serial_number = 12A3456
 standing_charge = 16.80
 unit_rate = 3.03
+# 1 for SMETS1 meters, 2 for SMETS2 meters
+meter_type = 1
+# volume correction factor, and calorific values used to convert SMETS2 consumption from m^3 to kWh
+# Defaults are typically used values. Your energy bill will show you the values used for your supply.
+volume_correction_factor = 1.02264
+calorific_value = 40