[MRG] Locality Sensitive Hashing for approximate nearest neighbor search(GSoC)

Before the beginning of the paragraph that explains how LSH Forest works, I would add a couple of sentences that introduces the motivation for LSH Forest variant, for instance:

"""
The requirement to tune :math:`k` for a given dataset makes classical LSH cumbersome to use in practice. The LSH Forest variant was designed to alleviate this requirement by automatically adjusting the number of digits used to hash the samples.
"""