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've no objection to keeping it in one place. I do, however, object to a function with two fairly distinct  purposes/phases called for two reasons and controlled via a boolean keyword arg. The big if statement is a good indicator that this needs breaking into two bits.