Forecast/observation reference time

This PR interprets the `significanceOfReferenceTime` for GRIB2 messages.
I would like to be careful about the difference between 'Analysis' and 'start of Forecast'

I would have thought that 'Analysis' was closer to CF's forecast_reference_time than 'start of Forecast'

Do we have example data with 
significanceOfReferenceTime == 0?
significanceOfReferenceTime == 1?