Refactoring problems.

examples/advection/advectionProblem.h

examples/heat-equation/tnl-heat-equation-eoc.h

examples/heat-equation/tnl-heat-equation.h

examples/inviscid-flow/1d/eulerProblem.h

examples/inviscid-flow/2d/eulerProblem.h

examples/mean-curvature-flow/tnl-mean-curvature-flow-eoc.h

examples/mean-curvature-flow/tnl-mean-curvature-flow.h

examples/navier-stokes/navierStokesSolver.h

src/TNL/CMakeLists.txt

src/TNL/Problems/CMakeLists.txt

src/TNL/Problems/HeatEquationEocProblem.h

src/TNL/Problems/HeatEquationEocProblem_impl.h

src/TNL/Problems/HeatEquationEocRhs.h

src/TNL/Problems/HeatEquationProblem.h

src/TNL/Problems/HeatEquationProblem_impl.h

src/TNL/Problems/MeanCurvatureFlowEocProblem.h

src/TNL/Problems/MeanCurvatureFlowEocProblem_impl.h

src/TNL/Problems/MeanCurvatureFlowEocRhs.h

src/TNL/Problems/MeanCurvatureFlowProblem.h

src/TNL/Problems/MeanCurvatureFlowProblem_impl.h