Bug fix and speed improvements

A buffer overflow bug was fixed.

Moreover, the Unweighted Unifrac has been significantly sped up and there has been additional CPU speedups across the board, too.

What's Changed

• Add precomputed sums for Unweighted + bug fix by @sfiligoi in #13
• Add tiling to speed CPU code by @sfiligoi in #14

Full Changelog: v1.1.2...v1.1.3