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.
I took them out. Given we don't have a representation for this metadata (if the comment is to be believed) and they default to zero, I thought it better not to set them. I could change it 255.