CMB coupling added

1) IC producer mhd_init.f90 is corrected for CMB coupling.
2) fine_velocity_mhd.f90 is corrected for CMB coupling.
3) add -DCMB_coupling flag in COMPILE_mhd_init.csh 
4) add -DCMB_coupling flag in makefile_mhd

Test result:
1) something wrong with -DREAD_SEED.  It takes 20000 time step to reach only z=0.2.  Thus for the test I removed -DREAD_SEED, the two simulation share the same IC but different random seed.
2) the two simulation have perfect same power spectrum for both DM and gas, even at redshift 150.
3) no difference could be seen in the slices
4) The simulation began from z=200.  It only take 7 steps to reach z=150.  Within the 7 steps, E_k ~10^-6, E_thermal ~ 10^-9.  This explains no difference in slice and power spectrum.