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.
Might be useful, as the `bounds` are not as one might expect i.e. `(lower, upper)` but they've been concatenated together into one bounds blob ...