Add documentation with doxygen

I added a configuration file for doxygen called Doxyfile and a CMake target called doc which, when built, generates a source code documentation in HTML and LaTeX in the folder documentation. I also added appropiate doxygen comments to all files execpt main.h, main.c and the tests. If everything works as planned the documentation will also be available online with the help of GitLab pages. The URL should be http://openpixi.gitlab.io/openpixi_c/.