Refactoring mean curvature flow.

...ar-diffusion-operators/tnlFiniteVolumeNonlinearOperator.h

...ffusion-operators/tnlFiniteVolumeNonlinearOperator_impl.h

...ear-diffusion-operators/tnlOneSideDiffNonlinearOperator.h

...iffusion-operators/tnlOneSideDiffNonlinearOperator_impl.h

src/operators/diffusion/tnlNonlinearDiffusion.h

src/operators/diffusion/tnlNonlinearDiffusion_impl.h

src/operators/operator-Q/tnlFiniteVolumeOperatorQ.h

src/operators/operator-Q/tnlFiniteVolumeOperatorQ_impl.h

src/problems/tnlMeanCurvatureFlowEocProblem.h

tests/unit-tests/CMakeLists.txt

tests/unit-tests/operators/CMakeLists.txt

tests/unit-tests/operators/fdm/CMakeLists.txt

tests/unit-tests/operators/fdm/tnlFiniteDifferencesTest.cpp