Use mathjax to render formulas in doxygen

Goal: Make formulas look nicer in doxygen.

• Add doc/doxygen/Doxylocal +USE_MATHJAX = YES
• Get rid of \textup, \textbf and \bf as they are not properly parsed by mathjax and should not be used in the formulas anyway
• See if further commands are not parsed properly

Spin-off of the discussion in !1964 (merged)