get_afni_model_PRF_DN

** usage: get_afni_model_PRF_DN NT  A0 x y Sig0 B0   A1 Sig1 B1

   This is mean to compute a model function for Conv_PRF_DN, given
   NT and a set of parameter inputs.

   consider something like:

       setenv AFNI_CONVMODEL_REF conv.ref.GAM.1D
       setenv AFNI_MODEL_PRF_STIM_DSET stim.144.LIA.bmask.resam+orig
       setenv AFNI_MODEL_PRF_ON_GRID NO

       setenv AFNI_MODEL_PRF_GAUSS_FILE gauss_dset

       # params: A0 X Y Sig0 B0  A1 Sig1 B1
       set nt = `3dinfo -nt $AFNI_MODEL_PRF_STIM_DSET`
       get_afni_model_PRF_DN $nt 5.9 -0.6 0.1  0.3 4.9  2.3 0.5 -1.4