GRIB deferred loading with Biggus.

This PR incorporates Biggus deferred loading within the GRIB loader.

This brings the GRIB loader in line with the deferred loading capabilities of the other primary data formats supported by Iris i.e. NetCDF, PP and Fieldsfiles.

 - [x] Rebase required since #971 has been merged.
If you expose henceforth as a staticmethod (or just a function) both the deferred and non-deferred loading components can use the same code path.