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.
@esc24 Might want to highlight to the reader here that `_reshape_vector_args` doesn't guarantee that the reshaped vectors are broadcastable ...