Document somewhat arbitrary 'proximity' values, and develop a plan for removing them.

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In trying to address #44, it becomes clear that right now we have fairly arbitrary "proximity" values. Some places, the value is 3.0, some 1.0, some 0.5. These are all temporary placeholders for "nearby" with various nuances (nearby stuff I can probably see, nearby stuff I can probably reach, nearby stuff I can probably hear) which should be represented explicitly.

• [ ] "reach" should be a property of an actor, and should probably default to 0.5 meters or so.
• [ ] CanSee itself should not automatically query the entire simulation graph, users should have some level of visual attenuation even in a straight line, and they should not be able to see around corners or through solid doors.
• [ ] perhaps preceding the previous two things, there should be utility functions for constructing common types of queries based on an actor, "can I reach it", "can I see it", etc, so that the actor's properties may be considered when doing the query itself.