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 did remove it, but added it back when I added the type casting truncation handling. I don't want to put it in the try as I believe firmly in keeping the code in the try as small as possible.