v0.5.2

PicoTree v0.5.2 changes:

• SearchKnn and SearchAknn performance increased. A priority queue is now maintained using an Insertion Sort instead of a Max Heap. The sorted queue resulting from Insertion Sort performs faster than the unsorted Max Heap in practice.
• Sort option removed from SearchKnn and SearchAknn. The output is now always sorted.
• EigenAdaptor now explicitly disallows fixed size matrices.
• Updated the Eigen example.