addition of usearch 6.1 wrappers for otu picking and renaming of qiime otu picking workflows

This is a big change set that @walterst and I have been working on. His recent PR which included his portion of these changes (#844) but wasn't updated to the lastest master from qiime/qiime, so a lot of tests failed. This PR includes his changes and mine.

For the record, the way we worked on this was #walterst worked on his changes in a branch in his fork (walterst/qiime). I added that fork as an upstream on my fork, and pulled his changes from there to get access to them without going through qiime/qiime, as neither of our changes were ready to go there yet. This worked out really well.