adding depth_registered_filtered injection

This enables the "injection" of a depth image filter into the middle of the rgbd processing pipeline. This is useful for efficiently removing known structure from the point cloud like a robotic manipulator.

It's done by defining the "depth_registered_filtered" argument, which then gets used by the point cloud processing node instead of the raw "depth_registered" topic.
I think the default value should read $(arg depth_registered)