RadioMedium: added NeighborCache support.
RadioMedium: sendToNeighbors() gets non-const transmitter.
GridNeighborCache: added implementation.
NeighborListCache: added implementation.
QuadTreeNeighborCache: added implementation.
NeighborCaches & IdealMac & AODVRouting (NED): merge fixes.
NeighborCaches: removed scenario-specific default values.
NeighborCaches: added examples.
util/QuadTree: added implementation.
QuadTreeNeighborCache: separated the data structure.