Accelerate AffinityPropagation.

Accelerate AffinityPropagation for large inputs using standard tricks
(in-place operations and out parameters) to avoid extra allocations.

Basic tests on datasets for 1000 points in 2D show ~10-20% increase in
speed.
AffinityPropagation: save memory by reusing tmp.
Merge pull request #3871 from anntzer/fast-affinity-propagation

[MRG+2] Accelerate AffinityPropagation.