HOURLY_GAS_METER_READING is really 5-minute reading with DSMR 5

[matthijskooijman]

In DSMR 4/5, the gas meter reading was transmitted to the electricity meter every hour, so the value updates only hourly. With DSMR 5, this frequency was increased to every five minutes, but the obis id was kept the same. This makes the HOURLY_GAS_METER_READING field name incorrect.

Maybe it should be generalized to just GAS_METER_READING (there is also a timestamp to tell you exactly how old the reading is), or maybe the name could be changed on the telegram specification version used?