Add new configure flag HYTEG_BUILD_DOC

By default this is ON and CMake file try to locate doxygen and add a custom target for building the documentation. So default behaviour is as before. Concept follows that used in waLBerla.

Rational: We do not always want/need to build the documentation and not on every machine doxygen will be available. E.g. CMake fails to locate it on HAWK.