Vector rules fns

Submitting  for comment : **not for merge yet**

Some tasks remain :

Need to ensure these changes don't impact load speeds adversely.

The main test for this "test__convert_vector_time_coords.py" is pretty messy.
The mechanics are largely tested by the separate tests for the support routines, but the main routine needs "something":  I can't quite get my head around testing "only" the relevant parts of it's behaviour, separate from the "generic vectorised function".

We still need to add at least the vector height code before we can really take advantage of this.
At first glance, I don't see any special extra complication with that, so hopefully it is a straightforward development from what we have here.