Proof of concept area weighted vertical regridding

[stephenworsley]

📰 Custom Issue

Investigate a solution to #5035.
Demonstrate the viability of expanding area weighted regridding to support non-horizontal coordinates. Initially, just involve an option to regrid with just a single, non-horizontal coordinate, but if viable, exploring the possibility to regrid over an arbitrary number of dimensions could be explored. API would not be set in stone as part of this investigation and may be explored in a further piece of work.