AC-FOVE: Generalized Aggregation Parfactor conversion

* AggregationParfactor: observation to be checked on getPRVs method
* GeneralizedAggregationParfactor: conversion to parfactors
* Parfactor: documentation
* GeneralizedAggregationParfactorTest and Pool: corrections on tests

Conversion to parfactors is pretty much the same as Aggregation
Parfactor's. We need to take care on the order of PRVs inside factors,
since they involve the parent and context variables.