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

@@ -71,6 +71,9 @@ def store_series(connection, series, metrics, rate_data):
 
     def fields_for_measurement(measurement):
         consumption = measurement['consumption']
+        conversion_factor = rate_data.get('conversion_factor', None)
+        if conversion_factor:
+            consumption *= conversion_factor
         rate = active_rate_field(measurement)
         rate_cost = rate_data[rate]
         cost = consumption * rate_cost
@@ -149,6 +152,9 @@ def cmd(config_file, from_date, to_date):
 
     g_mpan = config.get('gas', 'mpan', fallback=None)
     g_serial = config.get('gas', 'serial_number', fallback=None)
+    g_meter_type = config.get('gas', 'meter_type', fallback=1)
+    g_vcf = config.get('gas', 'volume_correction_factor', fallback=1.02264)
+    g_cv = config.get('gas', 'calorific_value', fallback=40)
     if not g_mpan or not g_serial:
         raise click.ClickException('No gas meter identifiers')
     g_url = 'https://api.octopus.energy/v1/gas-meter-points/' \
@@ -184,6 +190,8 @@ def cmd(config_file, from_date, to_date):
                 'gas', 'standing_charge', fallback=0.0
             ),
             'unit_rate': config.getfloat('gas', 'unit_rate', fallback=0.0),
+            # SMETS1 meters report kWh, SMET2 report m^3 and need converting to kWh first
+            'conversion_factor': (g_vcf * g_cv) / 3.6 if int(g_meter_type) > 1 else None,
         }
     }