reanalyze PCoA by selecting metadata categories

[antgonza]

Allow the user to select some groups in a give metadata category and restart the analysis with those samples.

[adswafford]

This is also now handled by filtering samples, but if there is a way to integrate filtering directly from Emperor, that'd be great. @ElDeveloper

[antgonza]

Nop, this is to only plot based on a given set of categories.

[antgonza]

closing as this is a subset of https://github.com/biocore/qiita/issues/2410

[antgonza]

closed the wrong issue ... 😄