Forecast/observation reference time

This PR interprets the `significanceOfReferenceTime` for GRIB2 messages.
These are optional octets from section 1 (22-nn). 

We don't have GRIB2 data coverage for this, nor can I identify the associated `gribapi` keys. I wonder if they've been implemented by ECMWF?

I've raised a support call with them seeking clarification ... otherwise I'll add a comment to state that we're explicitly defaulting to Gregorian.