Refactoring solvers.

src/TNL/problems/cfd/CMakeLists.txt

src/TNL/problems/cfd/navier-stokes/CMakeLists.txt

src/TNL/problems/cfd/navier-stokes/tnlNavierStokesSolver.h

...L/problems/cfd/navier-stokes/tnlNavierStokesSolver_impl.h