Cleanup/use mathjax
What this MR does / why does DuMux need it:
Use MathJax to render equations for doxygen. I also replaced \textup
with \text
for now
I do not see a problem with \textbf
and \bf
as they are listed as supported by mathjax (http://docs.mathjax.org/en/latest/input/tex/macros/index.html)
But mabye this is also related to browser versions.
Runtime reduces to 1m45.409s on my machine (without tests already), compared to 3m58.628s (without tests but with images) or 8m29.095s (with all tests and images)
The only downside I see when using MathJax is that MathJax actually takes some time to render once I click on a page.
Edit: The goal was to make formulas look nicer in doxygen.

Add doc/doxygen/Doxylocal +USE_MATHJAX = YES

Get rid of \textup
, and\mbox{\bf}
as they are not properly parsed by mathjax 
Further commands that are not parsed properly were not found
\textbf
stays for now.
Which issue this MR fixes: fixes #882 (closed)
Special notes for your reviewer: Needs to be merged after !2141 (merged)