Doxygen documentation needs review

[orlandini]

When trying to generate the Doxygen documentation from NeoPZ's root directory with the following command:
doxygen Doxyfile
Many errors were produced that prevented the ../neopz_html/html/index.html of being generated.

I've setup the Doxyfile to write warning logs on ../neopz_html/doxygen_log.txt. Many are the warnings, so I've silenced for now many of them that are not strictly necessary to be fixed at the current moment.

As a first effort in fixing this issue, I suggest searching the logs for the following terms:

• reached end of file while inside a f$ block
  This warning means that a LaTeX formula inside the documentation was not properly closed. Its correct syntax is \f$ latex-code-here \f$, not $ latex-code-here $.
• end of file while inside a group
  Probably there are issues regarding group openings ({@) and group closings (@}).

As discussed with @philippedevloo , I've tagged here people involved in some already identified projects that need fixing.