CMake Interaction Update

Closes #6 (closed) , #8 (closed) .

• Allow the user to specify the Python interpreter to be used by the pystencils-sfg CMake module via a hint variable to the find module
• Allow the user to pass custom command line arguments to generator scripts through CMake
• Move the default output path for generated files to ${CMAKE_CURRENT_BINARY_DIR}/_gen/<target>/gen
• Allow the user to customize the output directory