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/...
Could you add a test for the unrecognised case -- e.g. feed it a cell method with a valid (but unsupported) method + check that this key is set to 'missing'.