Vector pp rules for time coordinates

Replacement for @pp-mo's #1239. This builds on #1239 but reinstates the scalar version of the time pp rules. The vector version is not used (yet). I've removed some of the previous unit and tests and added others.
Oddness. I don't understand ...

`fp_coord` is 1-d, how can it possibly be spread over two dimensions?

Shouldn't `fref_time_coord` (which has a single value) be scalar with a dimension of `None` ... ?