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