check whether max. number of saved sessions is reached
        if: remove the oldest session

def checkMaxSessions(self, nMax=None):
        """
        check whether max. number of saved sessions is reached
        if: remove the oldest session
        """
        if nMax is None:
            nMax = self.opts['maxSessions']
        l = self.stateNames()
        if len(l) > nMax:
            for f in l[:len(l) - nMax]:
                self.tmp_dir_session.remove(str(f))