Paczynski Grey Atmosphere

When atm_option == 'paczynski_grey', MESA…

• DOCUMENTATION NEEDS TO BE FLESHED OUT

• ‘old’ description: create an atmosphere for given base conditions. inspired by B. Paczynski, 1969, Acta Astr., vol. 19, 1. takes into account dilution when tau < 2/3, and calls mlt to get gradT allowing for convection in atmosphere.