Improved time handling for PDT0

Adds support for significanceOfReferenceTime = 2, and fixes the handling of time within PDT0 to allow for the "reference time" coordinate being supplied as either "time" or "forecast_reference_time".
But bounds are* supported in `validity_time_coord`...

\* or at least [they claim to be](https://github.com/SciTools/iris/blob/master/lib/iris/fileformats/grib/_load_convert.py#L1277-L1281).