Implementing operator functions.

src/functions/tnlBoundaryOperatorFunction.h

src/functions/tnlExactOperatorFunction.h

src/functions/tnlFunctionEvaluator.h

src/functions/tnlMeshFunction.h

src/functions/tnlMeshFunctionEvaluator.h

src/functions/tnlMeshFunctionEvaluator_impl.h

src/functions/tnlMeshFunction_impl.h

src/functions/tnlOperatorFunction.h

src/mesh/grids/tnlTraverser_Grid3D_impl.h

src/operators/diffusion/tnlExactLinearDiffusion.h

src/operators/diffusion/tnlLinearDiffusion.h

src/solvers/pde/tnlBoundaryConditionsSetter.h

src/solvers/pde/tnlBoundaryConditionsSetter_impl.h

tests/unit-tests/tnlApproximationError_impl.h