Summary : a solution like the 'STASH' attribute, to carry grib-specific parameter identity.

At present, if you want to save Iris data for which no known parameter code translation exists, it just gets a 0, so you can't save multiple such things to one file.

Likewise on load, an unknown parameter code just results in an 'unknown' or anonymous cube
-- and maybe we can even wind up merging things that should be kept separate ??

I'm imagining a specially recognised cube attribute, along the lines of 'STASH' for PP+FF encoding.
It needs to encode : (edition, discipline parameterCategory, parameterNumber). We can probably handle GRIB1 in that form somehow, not clear on detail yet.