Add new possible units.
        
        :arg units: units
        :type units: :class:`str`
        
        :arg factor: multiplication factor to convert new units into base units 
        :type factor: :class:`float`
        
        :arg latexrepr: LaTeX representation of units (if ``None``, use *units)
        :type latexrepr: :class:`str`

def add_units(self, units, factor, latexrepr=None):
        """
        Add new possible units.
        
        :arg units: units
        :type units: :class:`str`
        
        :arg factor: multiplication factor to convert new units into base units 
        :type factor: :class:`float`
        
        :arg latexrepr: LaTeX representation of units (if ``None``, use *units)
        :type latexrepr: :class:`str`
        """
        if units in self._units:
            raise ValueError('%s already defined' % units)
        if factor == 1:
            raise ValueError('Factor cannot be equal to 1')
        if latexrepr is None:
            latexrepr = units

        self._units[units] = factor
        self._latexrepr[units] = latexrepr