docs: def speed_average(Temperature,element,isotope):
      r"""This function calculates the average speed (in meters per second)
      of an atom in a vapour assuming a Maxwell-Boltzmann velocity distribution.
      This is simply
  
      sqrt(8*k_B*T/m/pi)
  
      where k_B is Boltzmann's constant, T is the temperature (in Kelvins) and
      m is the mass of the atom (in kilograms).
  
      >>> print speed_average(25+273.15,"Rb",85)
      272.65940782
  
      >>> print speed_average(25+273.15,"Cs",133)
      217.938062809
  
      """
      atom = Atom(element, isotope)
      return sqrt(8*k_B*Temperature/atom.mass/pi)