get_afni_model_PRF_6

** usage: get_afni_model_PRF_6 NT A x y sigma sigrat theta

   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

       # A=2, x=0.2, y=0.5,  sigma=0.05  sigrat=5  theta=PI/8=0.3927
       set nt = `3dinfo -nt $AFNI_MODEL_PRF_STIM_DSET`
       get_afni_model_PRF_6 $nt 2 .2 .5 .05 5 0.3927