Small change to boolean.py to handle the case where a Boolean node ex…

…ists but doesn't have an update rule (e.g., a drug node that is turned ON when the drugged is applied)

pysb/importers/boolean.py

@@ -79,7 +79,7 @@ def __init__(self, filename, format='BooleanNet', mode='GSP', force=False):
             nfired_pat = []
         #~~~~~
         for name,state in self.initial_states.items():
-            rank = self.function_ranks.get(name)
+            rank = self.function_ranks.get(name, 1)
             # create monomer
             mon_sites = ['state']
             mon_site_states = {'state' : ['False','True']}