Area-weighted regridding for scheme-based regrid API

Does what the title says -- area-weighed regridding is now exposed in the new regrid API as a regrid scheme. The regridding itself is still carried out by `iris.experimental.regrid_area_weighted_rectilinear_src_and_grid`, this just exposes the functionality here using the new regrid API. Due to the similarities in the two cases, the work here is very similar to the work already completed in #1162 for Linear Regridding.
Rubbish! I was sure I'd got rid of that... In fact I had, but I might not have saved the resultant change before committing and pushing :grimacing: