Implementing total variation minimization test.

src/functions/tnlOperatorFunction.h

...erators/diffusion/tnlOneSidedTotalVariationMinimization.h

src/operators/tnlExactOperatorComposition.h

src/operators/tnlFunctionInverseOperator.h

src/operators/tnlNeumannBoundaryConditions_impl.h

src/operators/tnlOperator.h

src/operators/tnlOperatorComposition.h

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

...s/unit-tests/operators/diffusion/tnlLinearDiffusionTest.h

...ors/diffusion/tnlOneSidedTotalVariationMinimizationTest.h

tests/unit-tests/operators/tnlPDEOperatorEocTest.h