Add nixio as extra requirements for neo installation

code/requirements.txt

@@ -1,6 +1,6 @@
 # Plotting module required for example scripts
 matplotlib>=3.0.3
 
-neo>=0.9.0, <0.11
+neo[nixio]>=0.9.0, <0.11
 elephant>=0.9.0, <0.12
 odml>=1.5.1, <1.6