Forecast/observation reference time

This PR interprets the `significanceOfReferenceTime` for GRIB2 messages.
@marqh I've only seen `significanceOfReferenceTime` values of `0` and `3` from our data sets ...

The value of `3` is related to PDT 31 (satellite product) data. This is an `Observation time` so it makes sense (to me) that the `Reference time of data` maps to a CF `time` coordinate.