Clears all the cached information about this ruler.

def clear( self ):
        """
        Clears all the cached information about this ruler.
        """
        self._minimum   = None
        self._maximum   = None
        self._step      = None
        self._notches   = None
        self._format    = None
        self._formatter = None
        self._padEnd    = 0
        self._padStart  = 0