Add uchime2_denovo to close #92

[colinbrislawn]

WIP to close #92
Add internal functions and tests to use 'uchime' 'uchime2' or 'uchime3'
Open questions:

1. Externally, do we want to present all these as new methods/functions or new settings?

qiime vsearch uchime-denovo
qiime vsearch uchime2-denovo
qiime vsearch uchime3-denovo
# or
qiime vsearch uchime-denovo --p-method 'uchime2'

2. Internally, do we want to combine some of these? They are very similar, especially uchime2 and 3

The only difference [in --uchime3_denovo] from --uchime2_denovo is that the default minimum abundance
skew (--abskew) is set to 16.0 rather than 2.0.

3. Do we want to expose --abskew for some or all of these?

[hagenjp]

Hi @colinbrislawn,
1./2. We all agree that new parameters would be best vs. new methods. Thank you!
3. We do not have strong feelings about --abskew but making sure that the default is none (would be best so that it can match the algorithm by default)

[colinbrislawn]

new parameters would be best

Cool!

How does this look for the CLI? (CLI docs for the existing function)

qiime vsearch uchime-denovo \
  --p-method 'uchime2' \
  --p-mindiffs 99 # ignored when running uchime2 and uchime3
  --p-mindiv 0.8 # ignored when running uchime2 and uchime3
  --p-minh 0.99 # ignored when running uchime2 and uchime3
  ...

What should we do if someone passes settings that are not used by uchime2 and uchime3?
vsearch simply ignores them silently, which I don't love for our API

[colinbrislawn]

Let's add support for --abskew in a different PR, to keep things tidy ✨ 🧹

[colinbrislawn]

Is there a better way to do this?

[colinbrislawn]

Is --p-method a good name? I could go back to --p-algorithm or something new.

The methods are tested with 100% coverage, but the results are not, as I'm not sure how they should work with non-trivial examples.

If we want to build a test that shows the difference between these methods, here's commentary on vsearch's implementation:
torognes/vsearch#283
torognes/vsearch#503

@nbokulich, if you have the time and interest, I would appreciate your review!