Reference: https://stackoverflow.com/a/23766303
Documentation: https://cmake.org/cmake/help/latest/command/install.html#installing-directories

Added macro TNL_MPI_PRINT_COND for conditional print outs.

Reimplemented Config::ConfigDescription and Config::ParameterContainer using std::vector instead of TNL::List

This is much safer and simpler because it solves many problems due to
cyclic inclusion of TNL headers.

- There is an RAII wrapper called ScopedInitializer to handle
  finalization due to C++ exceptions and to avoid calls to Finalize in
  all exit branches.
- Fixed signature of MpiCommunicator::Init to match that of MPI_Init.

Added assertions to MpiCommunicator.h checking that MPI functions are not called with the null group

Fixed definition of static members in communicators when the header files are used in multiple translation units

Build of tools can be turned-off. Grid traversers for 1D and 2D has been rewritten using subdomain overlaps.