Add support for removing beliefs

The [`BeliefBase`](https://github.com/agentsoz/jill/blob/06f7e416b9e2677d51bdbd82a073ffd6623293e7/jill/src/main/java/io/github/agentsoz/jill/core/beliefbase/BeliefBase.java) class  does not currently support deletion/removal of beliefs. 