docs: 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