Revert "Merge branch 'feature/py-improve-parallel-communication-interface' into 'master'"

This reverts merge request !1230 (merged)

!1239 introduces a new dependency on mpi4py in the C++ part without CMake feature test. HAVE_MPI true doesn’t mean that the header mpi4py.hh is available. Breaks downstream code. Before !1230 (merged) was not a build dependency and was only needed at runtime.