MPI VAMPyR

[robertodr]

• Compile VAMPyR with MPI enabled. The CMake flag is -DVAMPYR_WITH_MPI=ON
• Allow to pass MPI communicator from Python to C++ I have used the technique shown here with pybind11 type casters.
• Bind MPI functions in MRCPP
• Add a test of MPI communication of trees
• Expand CI to test with and without MPI enabled