Consolidated pruning functions under an umbrella pruning dispatch function named 'LevelSetTree.prune'. Also removed the poorly designed root pruning function and reordered the LevelSetTree methods so that the straightforward ones are at the top.
Consolidated all utility functions into one file (debacl_utils.py). Added function 'plotForeground' which is a more user-friendly way to show foreground cluster labels in feature space.
Added a soup-to-nuts level set tree construction function. It constructs the similarity graph (knn), estimates the density (knn), builds the tree, and prunes the tree (mergeBySize).
continuing code cleanup of version 1.1