Use skbio's alpha diversity metrics
Remove qiime.pycogent_backports.alpha_diversity

You can dance if you want to
Add links to scikit-bio alpha diversity docs

Thanks for suggesting this @ElDeveloper!
replace more observed_species refs to observed_otus
Merge pull request #1542 from ElBrogrammer/brah-i-alpha-diversity

Remove PyCogent alpha diversity dependency