docs: def add_priorfactor(self,**kwargs):
          """Adds given values to priorfactors
  
          If given keyword exists already, error will be raised
          to use :func:`EclipsePopulation.change_prior` instead.
          """
          for kw in kwargs:
              if kw in self.priorfactors:
                  logging.error('%s already in prior factors for %s.  use change_prior function instead.' % (kw,self.model))
                  continue
              else:
                  self.priorfactors[kw] = kwargs[kw]
                  logging.info('%s added to prior factors for %s' % (kw,self.model))