Support for grib gdt5 saving.

This is for variable-resolution, rotated-pole data.
There is a serious bug in the gribapi preventing us from handling this template properly, so the results will not be useful until that is fixed : see https://github.com/pp-mo/iris/blob/grib_save_gdt5/lib/iris/fileformats/grib/_save_rules.py#L220 and https://software.ecmwf.int/issues/browse/SUP-1096
Don't quite get it...
The sense was intended to be that it if we _were_ to save irregular lat-lon data, then it would (or "should") be under GDT3.4, except we don't yet support that.
In what sense can you generalise that?