added filter to exclude external libraries

The necessary to_json(...) function are used. One can create a json
object by e.g. "nlohmann::json ttjson = nlohmann::json(tt)" with tt
being a timing tree

Part of issue #48

Vector2, Vector3 and Matrix3 define their own MPITrait::type functions that return an MPI_Datatype for them. It is created the first time the function is called and put into a static variable. It is therefore destroyed after the main() function exits, which is usually after MPI_Finalize has been called. Freeing the MPI_Datatype is therefore not possible and mpi::datatype cannot be used.

Also adds OpenMP support for macOS (requires Xcode 7.0, CMake 3.12 and separate libomp from MacPorts/Homebrew)