Handle targets that appear and disappear over time.

[Mark2000]

Relevant to the 1.0.0 release. Currently target opportunities are only calculated initially for known targets. Should be able to handle targets that become known later.

[Mark2000]

Current solution seems to be to add all known targets at start of episode, then filter the access based on timing/knowledge. This does prevent targets from being created due to a satellites actions (is there a use case for this). Probably a low priority issue for now, but will keep open since it is a nice-to-have.