AC-FOVE: refactoring for AggregationParfactors (split and
multiplication)

* StdParfactor, Parfactor: changed multiplicationHelper method name
* Factor: corrected method isConstant(), added method isEmpty()
* And, Or, Operator: getTypeArgument(), method to verify the argument
type and allow the use of wildcards in attributes
* Lists: difference() and new constructors
* AggParfactor: implementation of standard aggregation parfactors,
split and multiplication
* AggregationParfactor: new interface to define aggregation parfactors
specific operations
* AggParfactorTest: tests for multiplication