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.
I couldn't figure out a clean way to do this and if we drop the `_collapse_degenerate_points_and_bounds` collapse call this test will become redundant/change.