Relative path option for input_pathname in namelist definition doesn't work because the namelist XSD doesn't allow colons

In namelist_definition XML files nmlgen.py allows input_pathname to start with "rel:" with the end being a namelist variable that the path will be relative to. However, the XSD file for namelist doesn't allow this because it says input_pathname should be xs:NCName which is xs:Name without allowing colons.

Relative paths aren't currently being used (at least in CESM), but it does seem like a useful feature in some cases.

This is a really simple thing I could do a PR for.