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 think you can still get rid of the 'else' governing this code run -- doesn't add anything, but maybe simpler to look at.  Equally, you could include it at the end of the 'try' section.