SpatialGridVisitor: renamed visitor() function to visit().
SpatialGrid: passing Coord parameters as const Coord&.
IVisitor: added interface for data structure visitors.
QuadTree: QuadTreeVisitor is now inherited from IVisitor. Queries and visit() (visitor() renamed to visit()) are const functions.
SpatialGrid: SpatialGridVisitor is now inherited from IVisitor. Added getLineSegment() for lineSegmentQueries.
QuadTreeNeighborCache: followed recent QuadTree changes.
IObstaceLoss: passing Coord parameters by reference.
GridObjectCache: added implementation.
PhysicalEnvironment: added ObjectCache support.
ObstacleLoss: can use PhysicalEnvironment's object cache to compute total loss efficiently.