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)
Should really call this "missing", not invalid. (don't think there is any concept of "valid" values here)