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/...
I'm not quite happy with the generic error that occurs here if 'intervals' doesn't conform to expectations.  Maybe you could check that it splits as expected + raise a more informative error if not ?