Refactoring of handling of PDT in grib save code

This PR (hopefully) simplifies the code layout in `iris.fileformats.grib._save_rules` for the handling of he product definition template. It adds functions for each PDT number and removes some layers of complexity.
**...OR...**
On the other hand, virtually all the non-ecmwf data has both cutoff values = 0
I guess this may mostly be created here from UM output?