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
> I hadn't realised that bit 1 is MSB, I don't think I've ever seen that anywhere else.

What with that, the unusual negative representation, and the weird mix of scale/scaled and floating-point representations it's almost as if it were designed by committee. :wink: