docs: def __update_density(self, compound='', element=''):
          """Re-calculate the density of the element given due to stoichiometric changes as
          well as the compound density (if density is not locked)
  
          Parameters:
          ===========
          compound: string (default is '') name of compound
          element: string (default is '') name of element
          """
          _density_element = 0
          list_ratio = self.stack[compound][element]['isotopes']['isotopic_ratio']
          list_density = self.stack[compound][element]['isotopes']['density']['value']
          ratio_density = zip(list_ratio, list_density)
          for _ratio, _density in ratio_density:
              _density_element += np.float(_ratio) * np.float(_density)
          self.stack[compound][element]['density']['value'] = _density_element
  
          _density_lock = self.density_lock
          if not _density_lock[compound]:
              self.__update_layer_density()