docs: def charged(self, charge):
          """
          Adds/subtracts electrons and returns a new object.
  
          :param charge: number of electrons to add
          :type charge: integer
          :returns: a spectrum with appropriately shifted masses
          :rtype: CentroidedSpectrum
  
          """
          result = self.copy()
          result.addCharge(charge)
          return result