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.
Tricky one. I think it is OK how it is implemented now as it would be nice to keep all the regularisation code in one location.

The GRIB loader is a quite a confusing place and is in desperate need of some love, but I actually think refactoring would be done better as part of an organised single effort, and this change to how we use `_regularise` is not making the current situation significantly worse so I suggest it stays as it is for now.