Fix usage of OpenMP on macosx

On macosx one needs a openmp installation which does not come with the system, at least the omp.h header file is missing. brew has a package libomp which installs the header and a library. Both are found by the cmake module FindOpenMP and properly used during the compilation and linking. In themoment there is still a runtime error when executing the L1 tracking with OpenMP support on macosx, so switch off OpenMP for the time being. The problem is investigated already. refs #2028.

Cleanup OpenMP handling in the build system. Move check for OpenMP to top level. Remove OpenMP usage from all libs were it is not needed.