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`)

app/octopus_to_influxdb.py

@@ -102,7 +102,7 @@ def store_series(connection, series, metrics, rate_data):
         time = period.datetime().strftime('%H:%M')
         return {
             'active_rate': active_rate_field(measurement),
-            #'time_of_day': time,
+            'time_of_day': time,
         }
 
     measurements = [