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.
There's major duplication between this test and `test_t1_list` ... are they easy to combine? or use some commonality?

This would help highlight their differences i.e. t2 is a scalar or an array ...