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.
The offset is from the start of the file isn't it? But once the GRIB API has ingested a message the file pointer will be left at the end of the message, so to get back to the start of the message to read it again you have to subtract the length of the message.