added cython to docs install requirements

Author: bb515

docs/requirements.txt

@@ -1 +1 @@
--e .
+-e .[docs]

setup.py

@@ -12,6 +12,25 @@
 extra_compile_args = ['-O3']
 extra_link_args = []
 
+
+install_requires = [
+ 'meshio',
+ 'numpy',
+ 'pyopencl',
+ 'scipy',
+ 'tqdm',
+ 'h5py',
+ 'sklearn',
+ 'tqdm',
+ 'sphinx_rtd_theme'
+ ]
+
+
+docs_extras = [
+ 'cython',
+ ]
+
+
 ext_modules = [
  Extension(
  "peripy.create_crack",
@@ -55,15 +74,6 @@
  'console_scripts': ['peripy=peripy.cli:main']
  },
  ext_modules=cythonize(ext_modules),
- install_requires=[
- 'meshio',
- 'numpy',
- 'pyopencl',
- 'scipy',
- 'tqdm',
- 'h5py',
- 'sklearn',
- 'tqdm',
- 'sphinx_rtd_theme'
- ]
+ install_requires=install_requires,
+ extras_require={'docs': docs_extras}
  )