Support PP cross-section height.

This PR adds an additional PP load rule to correctly translate cross-section height as a dimensional coordinate into an Iris cube.

Given a fully specified `lbcode` with a `iy` of `02` meaning `height above sea level` with units of `km` (see UMDP F3, section 4.2, 16 LBCODE) the associated point and bounds data is expected within the extra data PP payload.