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 disagree, the "missing" concept is what you get from the default template, which is in fact 255 for bytes. 
I'm not sure why this was done before, but I honestly think that setting these things to 0 is what we don't know the meaning of.
I also checked data in the dataZoo, and all the ecmwf "standard" outputs have these set to "missing", so I think it makes more sense to leave that in place.