Add script to filter Doxygen warnings

Add a list of expected warnings from Doxygen and a Python script to
filter out those warnings from the output from Doxygen.

In some cases, it would be relatively easy to fix the Doxygen
documentation so that we don't get these warnings.  However, in other
cases there are bugs in Doxygen where the code and documentation are
perfectly valid, and Doxygen gives an erroneous warning message.

Currently, CI uses another script to filter out warnings related to
lines of code which have not been changed.  However, this doesn't work
well because the line number reported for a warning is often not the
line where the problem lies.

The new script should be useful for CI, and also for checking changes
locally before running CI.

Author: John Nonweiler