Refactoring the direct eikonal solver.

...al/Hamilton-Jacobi/Solvers/hamilton-jacobi/CMakeLists.txt

...acobi/Solvers/hamilton-jacobi/tnl-direct-eikonal-solver.h

...obi/Solvers/hamilton-jacobi/tnlDirectEikonalMethodsBase.h

...olvers/hamilton-jacobi/tnlDirectEikonalMethodsBase_impl.h

...-Jacobi/Solvers/hamilton-jacobi/tnlDirectEikonalProblem.h

...bi/Solvers/hamilton-jacobi/tnlDirectEikonalProblem_impl.h

...on-Jacobi/Solvers/hamilton-jacobi/tnlFastSweepingMethod.h

...bi/Solvers/hamilton-jacobi/tnlFastSweepingMethod1D_impl.h

...bi/Solvers/hamilton-jacobi/tnlFastSweepingMethod2D_impl.h

...bi/Solvers/hamilton-jacobi/tnlFastSweepingMethod3D_impl.h

src/TNL/Problems/PDEProblem.h

src/TNL/Solvers/SolverStarter_impl.h

src/TNL/TypeInfo.h