RadioMedium: added NeighborCache support.

RadioMedium: display string fixes.
RadioMedium: sendToNeighbors() gets non-const transmitter.
NeighborCaches: removed scenario-specific default values.
NeighborCaches: added examples.
NeighborListCache: cosmetics.
NeighborListCache: fixed an uninitialized ptr.
QuadTreeNeighborCache: separated the data structure.
util/QuadTree: added implementation.