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

docs/source/usage/generator_scripts.md

@@ -320,6 +320,7 @@ with the `--help` flag:
 $ python kernels.py --help
 ```
 
+(custom_cli_args)=
 ## Adding Custom Command-Line Options
 
 Sometimes, you might want to add your own command-line options to a generator script