Return variables needed for vertical transforms

[lesserwhirls]

From e-support:

We use hybrid sigma-pressure coordinatres in our oerational simulations. The netcdf files conatin variables a, b a_half and b_half (see here). Is there any way to include these variables into any output of NetCDF subset, that contains 3D fields? Ideally, it would be great if vertical slicing could handle them properly.

[cwardgar]

See https://github.com/Unidata/thredds/issues/1048#issuecomment-375558122