Move pygraphviz to GUI module since it is not installable on cluster system

gui/setup.py

@@ -74,5 +74,5 @@ setup(
         'gui_scripts': ['cadbiom = cadbiom_gui.__main__:main'],
     },
 
-    install_requires=["cadbiom"],
+    install_requires=["cadbiom", "pygraphviz"],
 )

library/setup.py

@@ -137,7 +137,7 @@ setup(
     ext_modules=modules,
     description="Cadbiom library",
     long_description=open('README.md').read(),
-    install_requires=["networkx<2", "lxml", "pygraphviz", "pycryptosat"],
+    install_requires=["networkx<2", "lxml", "pycryptosat"],
 
     # Tests
     tests_require=['pytest'],