Fix FutureWarning in qiime.stats._perform_pairwise_tests
MAINT: update compare_categories.py to use latest skbio anosim and permanova API
MAINT: update compare_distance_matrices.py to use scikit-bio's mantel function with --method mantel
MAINT: update compare_categories.py --method mantel_corr to use scikit-bio's mantel function
update correlogram plotting to only plot p-vals that aren't None/NaN
remove old mantel code in favor of scikit-bio's mantel function
update docstring for mc_t_two_sample
ENH: method to compute a taxonomy based index
TST: tests for index computation
TST: added script usage test
ENH: allow for specifying an output column name
ENH: support for an arbitrary key
DOC: Fix group_significance.py Sphinx issues

Fixes #1467 #1615.
DOC: fix remaining Sphinx errors in doc build
Addressing @ElDeveloper's comments
Addressing more of @ElDeveloper's comments
Fixed spacing
MAINT: remvoing tests for supported versions for python packages. these are outdated, incomplete, and redundant (as the versions are already reported here)
ENH: version information for sumaclust and sortmerna now included, fixes #1735
BUG: split_libraries_fastq.py --phred_offset no longer ignored

Fixes #1656.