Add Cython as additional dependecy

setup.py

@@ -118,6 +118,7 @@ setuptools.setup(name='pystencils',
                      'autodiff': ['pystencils-autodiff'],
                      'doc': ['sphinx', 'sphinx_rtd_theme', 'nbsphinx',
                              'sphinxcontrib-bibtex', 'sphinx_autodoc_typehints', 'pandoc'],
+                     'use_cython': ['Cython']
                  },
                  tests_require=['pytest',
                                 'pytest-cov',