Added back speed_sound 
Added back speed_sound 

CoolProp/CoolProp-museum,CoolProp/CoolProp-museum,ibell/coolprop,ibell/coolprop,ibell/coolprop,CoolProp/CoolProp-museum,CoolProp/CoolProp-museum,CoolProp/CoolProp-museum,CoolProp/CoolProp-museum,ibell/coolprop,ibell/coolprop,ibell/coolprop,ibell/coolprop,CoolProp/CoolProp-museum,ibell/coolprop