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)
I don't think this test is wanted.
It is testing the properties of \_TIME_RANGE_UNITS, and there's really no reason we should care about the details of that, only what "time_range_unit()" does.
At present nothing else refers to it anyway (in fact, we could embed it in the function).