change default electrons from None to 0 for depository

Reactions expect electrons to be an integer, but the previous constructor for Depository reactions initialized it as None, which can lead to crashes

Author: jonwzheng

rmgpy/data/kinetics/depository.py

@@ -62,7 +62,7 @@ def __init__(self,
                  depository=None,
                  family=None,
                  entry=None,
-                 electrons=None,
+                 electrons=0,
                  ):
         Reaction.__init__(self,
                           index=index,