XneqY should not be consistent if X==Y

[Dagefoerde]

Consider the following minimal example:

x = new IntVar(store, 1, 100);
store.impose(new XneqY(x, x));

To the human reader, it is obvious that such a constraint cannot be satisfied. However, store.consistency() happily returns true, leaving x's domain intact. It might be sufficient to add a check for x != y to the consistency method in XneqY.

[radsz]

After some discussion we decided to throw a different type of the exception to let the user know that it is likely that the model is incorrectly constructed.