uwuw_preprocessor::check_material_props is not used anywhere

[helen-brooks]

The private method uwuw_preprocessor::check_material_props is not used anywhere. It's not clear to me which of the following apply:

1. this is dead code, or
2. it should be used somewhere, or
3. it should be public so that it is available for use.

Depending on which of the above apply, I propose one of the following fixes:
If it is dead code, then method should be deleted.
If it should be performing a check somewhere (even optionally) then add this feature.
If it is simply intended as a useful debugging tool, suggest that it made public and used in a test case with examples that both pass and fail test.

[ahnaf-tahmid-chowdhury]

The function uwuw_preprocessor::check_material_props validates material assignments for a given volume, reporting errors for missing or multiple material properties.

It seems, it is not added in unit tests.

[gonuke]

I think the best thing to do would be to make this method public and add a unit test.