Need xmllint Github Action

With the new multiple namelist definition file capability, there will soon be multiple namelist definition XML files, along with the CAMDEN registry XML file, that may not necessarily be checked during the current unit testing worfklow. Thus it could be beneficial to run `xmllint` on these files whenever they are modified (or a new one is introduced), similar to what is done when applying `pylint` to python files.

On top of that, having a first-step `xmllint` check before running more complicated software tests using Github Actions could help the CI system avoid any extra, wasted setup for tests that we know will just fail immediately due to an XML error.