Refactoring gradient norm operators.

src/functions/tnlDomain.h

src/operators/diffusion/CMakeLists.txt

src/operators/diffusion/tnlExactNonlinearDiffusion.h

src/operators/diffusion/tnlExactNonlinearDiffusion_impl.h

src/operators/geometric/tnlExactGradientNorm.h

src/operators/operator-Q/CMakeLists.txt

src/problems/tnlMeanCurvatureFlowEocRhs.h