Added dendrogram visualization
BUGFIX: Forgot to divide by the sum in order to get the average
First attempt to use SplHeap for heap_linkage
Comment additions to heap_linkage

Changed the child classes to use the distance matrix
directly and not through the heap.

Added MatrixHeap to the repository.
Removed associative heap from util

HeapLinkage now depends on MatrixHeap and not associative heap.
Comment additions in merge strategies
Merge branch 'spl-heap-linkage' into hierarchical-clustering