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
Note: arguably this change should be made in the 'old gribapi version' version of this file, too.
As I don't have any quick way of confirming that, and I doubt the relevance of still having this test for the older gribapi version, I'm not going to change that now.
It seems to cover a rounding/truncation issue, which we are now avoiding by setting with an integer instead of the float.