Additional Modifications for Negative Snow

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

[jules#167](https://github.com/MetOffice/jules/issues/167) introduces outstanding modifications to deal with the underlying problem of negative amounts of snow. The scientific details are recorded there. This issue covers only the modifications required to make those modificiations available within lfric_apps.

### Further details of the issue.

_No response_

Start by reading the scientific details in jules#167, then determine which lfric_apps integration points must expose those modifications. The issue names no files or tests, so the first task is locating the relevant snow implementation and its existing checks. Done means the negative-snow modifications from jules#167 are available within lfric_apps.

Written by the indexing model from the issue text.