correct for CMB coupling in fine_velocity_mhd.f90

correct for CMB coupling in fine_velocity_mhd.f90

If I understand correctly,  u(5) = 0.5mV^2 + U.
if z>150, u(5) = 0.5mV^2 + E_thermal
if z<150, u(5) = u(5)
Thus I just add one if sentence in it.
After that, the change in E_k is calculated as usual by using the CMB coupled u(5).
and then gaz(5) = gaz(5) + change in kinetic energy.

Please check that.