Addition/deletion of centroids tuning

I have improved methods of centroid addition and deletion.

* The evidence of removed centroid can be distributed to the nearest centroid (or k nearest centroids)
* New centroid is created based on position of picked centroid (or k centroids). Some tweaks are possible:
    * centroid may be picked with probability proportional to centroid's sigma or sigma * frequency
    * new centroid may be drawn from area around the center in various ways
* Letters experiment was extended. I run this experiment to find best method and optimal initial parameters.