Improved access control for World class, including preventing access to a non-immutable Zone array (!).
Removed obsolete InvalidLocationException class in favor of not being stupid when trying to assign a player to a location.
Improved test accuracy be reducing the allowable range of movement for NPC.
 Attempts to make tests STEP_LENGTH agnostic are proving somewhat futile, as most of the tests involving movement depend on a STEP_LENGTH of 0.2.  This is obviously poor test design.
Added WorldObjectTree class.
Added WorldObjectTree class.
Improved object addition and removal.