Renaming folder schemes to operators.

src/operators/diffusion/tnlExactLinearDiffusion.h

src/operators/diffusion/tnlLinearDiffusion.h

src/operators/euler/CMakeLists.txt

src/operators/euler/fvm/CMakeLists.txt

src/operators/euler/fvm/tnlLaxFridrichs.h

src/operators/gradient/CMakeLists.txt

src/operators/gradient/tnlCentralFDMGradient.h

src/operators/tnlFiniteDifferences.h

tests/unit-tests/CMakeLists.txt

tests/unit-tests/operators/CMakeLists.txt

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

...unit-tests/operators/diffusion/tnlLinearDiffusionTest.cpp

tests/unit-tests/operators/tnlPDEOperatorEocTestResult.h

tests/unit-tests/operators/tnlPDEOperatorEocTestSetter.h

tests/unit-tests/operators/tnlPDEOperatorEocTester.h

tools/src/tnl-init.h