Product definition template 1.

This PR extends the new GRIB2 capability to include product definition template 1.

I'm conscious that I've created utility functions:
 * `reference_time_coord`
 * `forecast_period_coord`
 * `validity_time_coord`

which could/may be more generic to accommodate future functionality. But since I don't have a use-case or pattern for *how* to make them generic, I'm leaving them as is and assuming that follow-on PRs will tend towards the *agreed* solution (@pp-mo)
Looking forward to PDT#8, the forecast period is then defined in a more complex way, which makes me doubtful about the whole idea of including format-specific detail in these coordinate-builder functions :  I think it would be better to have these routines more generic -- but that means removing statements about specific message keys here.