Create Diagnostic for Subclass Cycles

[ShaunRose]

If I were to declare the following:
(subclass a b)
(subclass b c)
(subclass c a)

There is a loop of subclassing
a-----
| |
L B |
| |
L C

;; Circular subclass reasoning.
(instance Class31-1 Class)
(instance Class31-2 Class)
(instance Class31-3 Class)

(subclass Class31-1 Animal)
(subclass Class31-2 Animal)
(subclass Class31-3 Animal)

(subclass Class31-1 Class31-2)
(subclass Class31-2 Class31-3)
(subclass Class31-3 Class31-1)

(instance Animal31 Class31-2)

(query (instance Animal31 Class31-1))

(answer yes)

I need to confirm this with Dr. Pease because one of the TQG31 tests does this.