Consistified GeoSpace and RealWorldUSLengthUnit naming to Reality–

The choice to renamed from a Geo– or RealWorld— prefix was made because the former sounds planet-based (like latitude and longitude) and the latter is clumsy.  Also, Reality– is a stronger reference to the AR concepts this'll be used for.

• Renamed GeoSpace to RealitySpace in model code, model filename, tests, and project file.
• Renamed RealWorldUSLengthUnit to RealityLengthUnit in model code, model filename, tests, and project file.
	» Notably, This is RealWorldUSLengthUnit moving away from its US Imperial Unit System-specifc focus to a more general unit focus in one class.  Meters and such should be added later.
	• Also renamed RealWorldUSLengthUnitTests to RealityLengthUnitTests.