Inconsistency for returned value type of maybe_equals()

[arcondello]

Originally reported by @JoelPasvolsky

Depending on whether it's tested on another symbol or on itself, maybe_equals() returns Boolean or integer:

>>> model = Model()
>>> i = model.integer(100, lower_bound=0, upper_bound=20)
>>> j = model.integer(3, lower_bound=-10, upper_bound=10)
>>> j.maybe_equals(j)
True
>>> j.maybe_equals(i)
0

[arcondello]

This should always return a number. Specifically 0 if not equal, 1 if possibly equal, and 2 if definitely equal.