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.
I think this is a fair assessment @bjlittle. It works and I don't find it too clunky (not more so than the rest of the GRIB loader) so a refactor can be left for another PR.