Test for new Doxygen warnings

[straitm]

We should have a build test that checks whether the branch adds new doxygen warnings. These usually happen because a new class/method/member has been added and not completely documented, or because doxygen commands have been misspelled or misplaced.

I currently do this by running make dox for master and the PR branch and diffing the outputs. An automated test would have to ignore the frequent case in which an existing undocumented object moves to a new line.