Add the collision group option to the UI for boid particle physics.

The option already works, just missing from the ui since it
has completely different option set from newtonian and fluid.

release/scripts/startup/bl_ui/properties_particle.py

@@ -716,6 +716,8 @@ class PARTICLE_PT_physics(ParticleButtonsPanel, Panel):
             col.prop(boids, "land_personal_space")
             col.prop(boids, "land_stick_force")
 
+            layout.prop(part, "collision_group")
+
             split = layout.split()
 
             col = split.column(align=True)