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
b55c78a289