Fix bug in parsing group adjacency list multiplicities.

Also added comments for the somewhat complicated reg-ex.
Thanks for finding the bug, Nick.

Author: rwest

rmgpy/molecule/adjlist.py

@@ -353,7 +353,11 @@ def fromAdjacencyList(adjlist, group=False, saturateH=False):
     if lines[0].split()[0] == 'multiplicity':
         line = lines.pop(0)
         if group:
-            match = re.match('\s*multiplicity\s+\[\d(?:,\s*\d)*\]\s*$', line)
+            match = re.match('\s*multiplicity\s+\[\s*(\d(?:,\s*\d)*)\s*\]\s*$', line)
+            # should match "multiplicity [1]" or " multiplicity   [ 1, 2, 3 ]" or " multiplicity [1,2,3]"
+            # and whatever's inside the [] (excluding leading and trailing spaces) should be captured as group 1.
+            # Multiplicities must be only one digit (i.e. less than 10)
+            # The (?:,\s*\d)* matches patters like ", 2" 0 or more times, but doesn't capture them (because of the leading ?:)
             assert match, "Invalid multiplicity line '{0}'. Should be a list like 'multiplicity [1,2,3]'".format(line)
             multiplicities = match.group(1).split(',')
             multiplicity = [int(i) for i in multiplicities]