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.
Yeah ... it seems odd that we're reshaping and broadcasting up from a specified shape and dimensionality on bootstrap, but we don't use that knowledge to reduce. I have concerns ...

We may just need to plug and play, to see how it behaves ...