Vectorise convertion of vertical coordinates

This PR extends the capability of the `pp_rules._convert_scalar_vertical_coords` function to cope with array data rather than scalar values.

Re-brands `_convert_scalar_vertical_coords` as `_convert_vertical_coords`.

Assumes that the following arguments will remain as scalar:
* lbcode
* lbvc
* stash

Assumes that the following arguments may be either scalar or array:
* blev
* lblev
* bhlev
* bhrlev
* brsvd1
* brsvd2
* brlev