Adding a new 3D stimulus
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*  Add a C routine **get_stim_3d_ppl_XXX** to <i>stim_util.c (.h)</i>

*  Add a call to the routine <i>get_stim_3d_ppl_XXX</i> in the
   routine **get_3d_stim_data** in the file <i>stim_util.c</i>