pyro - "custom wind volumes"

Sometimes you want to have pyro behave a certain way in a certain area. Eg.  the smoke should go from left to right, then suddenly shoot upwards after a few seconds.

Instead of fiddling with the advection or velocity updates, it's a bit simpler to create a separate pyro source that only emits velocity.

Simply create a source as you normally do, but remove it's density & temperature. Remember to scale these to zero in the DOP area too!