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.
Hmmm I'm assuming this would be a major problem if the reshaped vectors were not broadcastable ... the arithmetic operations would just cause a `np` exception ...