PCODAR: errode old objects and points

[kev-the-dev]

• If objects are currently within field of view, but have not been seen (new points associated) recently (configurable time), remove the object. This is useful, for example, when a kayak moves by the boat and then leaves.
• If an object is currently seen (in view and new points associated), remove older points associated with that object. This will allow the object to "move" over time if it drifts without "smearing"