Adding the PythonicAPI Extraction examples

src/Cxx.md

src/Cxx/PolyData/ExtractSelection.cxx

src/Cxx/PolyData/ExtractSelectionUsingCells.cxx

src/Cxx/PolyData/ExtractSelectionUsingPoints.cxx

src/Cxx/PolyData/MergeSelections.cxx

src/Python.md

src/Python/PolyData/ExtractSelectionUsingCells.py

src/PythonicAPI.md

src/PythonicAPI/PolyData/ExtractSelectionOriginalId.py

src/PythonicAPI/PolyData/ExtractSelectionUsingCells.py

src/PythonicAPI/PolyData/ExtractSelectionUsingPoints.py

src/PythonicAPI/PolyData/MergeSelections.py

src/PythonicAPI/PolyData/VertexConnectivity.py

.../Baseline/Cxx/PolyData/TestExtractSelectionUsingCells.png

...Baseline/Cxx/PolyData/TestExtractSelectionUsingPoints.png

...seline/Python/PolyData/TestExtractSelectionUsingCells.png

...e/PythonicAPI/PolyData/TestExtractSelectionOriginalId.png

...e/PythonicAPI/PolyData/TestExtractSelectionUsingCells.png

.../PythonicAPI/PolyData/TestExtractSelectionUsingPoints.png

...ing/Baseline/PythonicAPI/PolyData/TestMergeSelections.png