Make Communicator static.

src/TNL/Communicators/MpiCommunicator.h

src/TNL/Communicators/NoDistrCommunicator.h

src/TNL/Functions/MeshFunction.h

src/TNL/Functions/MeshFunction_impl.h

...NL/Meshes/DistributedMeshes/DistributedGridSynchronizer.h

src/TNL/Meshes/DistributedMeshes/DistributedGrid_1D.h

src/TNL/Meshes/DistributedMeshes/DistributedGrid_2D.h

src/TNL/Meshes/DistributedMeshes/DistributedGrid_3D.h

src/UnitTests/Mpi/DistributedGridIOTest.cpp

src/UnitTests/Mpi/DistributedGridTest_1D.cpp

src/UnitTests/Mpi/DistributedGridTest_2D.cpp

src/UnitTests/Mpi/DistributedGridTest_3D.cpp

tests/mpi/MeshFunctionEvauateTest.cpp