Bugfix in mutator when len(expr) == 1

gte/mutator.py

     return expr
 
 def mutupdtype(expr):
-    pos = randint(0, len(expr) - 1)
+    if len(expr) == 1:
+        pos = 0
+    else:
+        pos = randint(0, len(expr) - 1)
     optype = [RpnSymbol.OPERATION, RpnSymbol.VALUE, RpnSymbol.VARIABLE]
     del(optype[optype.index(expr[pos].optype)])
     expr[pos] = RpnSymbol.random(optype[randint(0,1)])