added is_cas(), check_cas()

ChemID.py

@@ -247,4 +247,22 @@ def name2cas(name):
             cas = possible_cas[0]
         except:
             cas = None
-    return cas
+    return cas
+
+def check_cas(cas):
+    n1,n2,n3 = cas.split('-')
+    # combine and flip first 2 numbers
+    tmp = ''.join([n1,n2])[::-1]
+    # sum of number*position in string
+    check = sum([i*int(tmp[i-1]) for i in range(1,len(tmp)+1)])
+    # mod 10
+    check = check%10
+    # if these match, then it's a legit cas number
+    return check == int(n3)
+
+def is_cas(cas):
+    try:
+        return check_cas(cas)
+    except:
+        return False
+