GRIB support for saving time increment for time aggregated data (PDT4.8)

This PR enhances the existing code for saving data in PDT 4.8. As well as refactoring the code it adds support for parsing and saving the cell_methods on cubes to determine the interval (e.g hourly or daily) of the data that has been summed/averaged/...
Can you explain why this is necessary ?
Presumably it upsets the gribapi somehow, but that seems surprising as it does use numpy (for example with vector keys like 'PV', 'longitudes' etc).