Overlapping spheres.

[ezorita]

Implements the following updates to the definition of Spheres clustering (requested in Issue #22):

Current behavior:

1. Sort by sequence count.
2. By seqcount order: if a sequence has not been claimed, it becomes a centroid. Otherwise continue to the next sequence.
3. The centroid claims all its hits that haven't been claimed yet by another centroid.

New behavior:

1. Sort by sequence count.
2. By seqcount order: if a sequence has not been claimed, it becomes a centroid. Otherwise continue to the next sequence.
3. The centroid claims all its hits that haven't been claimed yet by another centroid or all hits whose distance to the current centroid is less than the distance to its canonical.

Remarks:

The new algorithm is not so greedy and produces more meaningful spheres.

[ezorita]

Additionally we can also define a minimum rate to steal sequences from other spheres. This is to avoid tiny satellites to steal sequences from a big sphere, hence creating an independent (and probably wrong) cluster.