Refactor iteration over ISample tree: fix constness and provide preorder strategy

...nAPI/inc/vector_less__const_ISample_ptr___greater_.pypp.h

Core/PythonAPI/src/ICompositeSample.pypp.cpp

Core/PythonAPI/src/PythonModule.cpp

...PI/src/vector_less__const_ISample_ptr___greater_.pypp.cpp

Core/Samples/inc/ICompositeIterator.h

Core/Samples/inc/ICompositeIteratorStrategy.h

Core/Samples/inc/ICompositeSample.h

Core/Samples/src/ICompositeIterator.cpp

Core/Samples/src/ICompositeIteratorStrategy.cpp

Core/Samples/src/ICompositeSample.cpp

Core/Samples/src/ISample.cpp

Core/Samples/src/ISampleVisitor.cpp

Core/Tools/inc/SamplePreprocessor.h

Core/Tools/src/SamplePreprocessor.cpp

Tests/UnitTests/TestCore/MultiLayerTest.h