Constant velocity 3-D prestack migration

  This module defines a constant-velocity prestack migration operator. It requires spat_map=gather in the parameters.

• map_amp_init =  mig3d_const_init
• map_amp = mig3d_const

The interface of mig3d_const_init is:

function mig3d_const_init (slow_mig) result (stat)
real :: slow_mig
integer :: stat