Simulation: Add modifier to access simulation data

For now the "Simulation" modifier only exists for point cloud objects, because
we need this for the particle system. Right now, the modifier is doing nothing.

There is a new `DEG_add_simulation_relation` function that is used
by the modifier to make sure that the simulation is evaluated before
the modifier is executed.

Reviewers: brecht, sergey

Differential Revision: https://developer.blender.org/D7549

release/scripts/startup/bl_ui/properties_data_modifier.py

@@ -1826,6 +1826,10 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
         col.prop(md, "thresh", text="Threshold")
         col.prop(md, "face_influence")
 
+    def SIMULATION(self, layout, ob, md):
+        layout.prop(md, "simulation")
+        layout.prop(md, "data_path")
+
 
 class DATA_PT_gpencil_modifiers(ModifierButtonsPanel, Panel):
     bl_label = "Modifiers"