add normalization alternatives to rarefying (DESeq and metagenomeSeq's c...

...umulative sum scaling (CSS)) to Qiime as described by McMurdie and Holmes in "Waste not want not why rarefying microbiome data is inadmissible", PLoS CompBio, 2014.  We are wrapping R (which also has the test code), so installation of rpy2 is required.