It looks like we no longer support `horiz_winds` as a nudging variable

With the changes we made to support `U` and `V` as nudging fields it looks like we've lost support for `horiz_winds` as a nudging field. This is an issue for cases where the data used for nudging comes from older SCREAM runs and we don't have U and V as output, instead they are combined into horiz_winds.

Start by locating the nudging-field handling for U and V and compare it with the prior horiz_winds path. Reproduce the case using older SCREAM nudging data that provides horiz_winds rather than U and V; done means horiz_winds is accepted again without breaking the newer fields.

Written by the indexing model from the issue text.