MPITraits for std::complex

Nils-Arne Dreierrequested to merge
MPITraits_for_complex into master

This MR adds the MPITraits for std::complex<float>, std::complex<double> and std::complex<long double> that map to the MPI_Datatypes MPI_CXX_FLOAT_COMPLEX MPI_CXX_DOUBLE_COMPLEX and MPI_CXX_LONG_DOUBLE_COMPLEX.

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