Changes existing priorfactors.

        If given keyword isn't already in priorfactors,
        then will be ignored.

def change_prior(self, **kwargs):
        """
        Changes existing priorfactors.

        If given keyword isn't already in priorfactors,
        then will be ignored.
        """
        for kw in kwargs:
            if kw in self.priorfactors:
                self.priorfactors[kw] = kwargs[kw]
                logging.info('{0} changed to {1} for {2} model'.format(kw,kwargs[kw],
                                                                       self.model))