Fix broken multiclass() function for multi-multiclassing, i.e. calling multiclass() on something that is already a multiclass

[jroven]

In issue #1402, Jake and I got the multiclass module and tests almost completely working, but there is still a bug somewhere in the multiclass() function that results in an infinite loop. We unfortunately did not have time to debug this so we have created a backlog issue for the debugging of this function. The function multiclass() should be fixed so calling it on something that is a multiclass does not result in an infinite loop.