New feature: compilation of doc using latexmk

[pparys]

Hi there!
I improved the compilation process for latex files, using the "latexmk" command.
It is better because:

• Only files that were changed are compiled again, not all files.
• The number of runs of latex is adjusted automatically by latexmk (in our context usually one run is enough, unless there are some references in the text).

If the "latexmk" command is not available in the system, then the previous compilation functions are used.