Polish robust PCA

[TuomasBorman]

Improve the robust PCA. Especially, code and things discussed here: #804 and #789

[antagomir]

Cites the Gemelli repository on the manpage, the original source:
https://github.com/biocore/gemelli

[antagomir]

Revise the demo workflows and move to relevant location

[TuomasBorman]

Check the unresolved comments in #808

[TuomasBorman]

TODO:

• Add unit tests
• Tidy the example dataset. colData includes columns with names including many dots
• The taxonomy data includes features from multiple taxonomic levels. These should be separated into individual tables based on taxonomic level