def getParticleBest(self, particleId):
return self._particleBest.get(particleId, (None, None))<EOL>
Return the best score and position for a given particle. The position
        is given as a dict, with varName:varPosition items in it.

        Parameters:
        ---------------------------------------------------------------------
        particleId:    which particle
        retval:        (bestResult, bestPosition)