Standardize usage of AtomicInteger in Centroid

Mixing usage of incrementAndGet with getAndIncrement creates a race condition in which it's possible
for two Centroids to end up with the same ID. This can cause problems during merges, because the ID
is used as a "tiebreaker" in compareTo.
Merge pull request #27 from keenlabs/master

Standardize usage of AtomicInteger in Centroid