Added support for loading regular gaussian grids from GRIB files.

This PR adds the ability to load data defined on the `regular_gg` (regular gaussian) grid type from GRIB files. These changes only make it possible to load such data, saving back to grib2 format is somewhat non-trivial so I propose to leave it for a separate PR.

Related PRs:
SciTools/iris-code-generators#12
SciTools/iris-test-data#18

Remaining tasks:

- [ ] update iris-test-data reference when SciTools/iris-test-data#18 is merged