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.