Prepare for CRAN submission

[dmcglinn]

• run devtools::check() to get warnings and output that would make CRAN mad and fix accordingly
• update vignette to use new version of package
• consider depreciating some function names in favor of new names (e.g., get_delta_stats is not a good name)
• ensure tests are running properly and are as strong as they can be
• improve function documentation (e.g., rarefaction function)
• add warning for unbalanced designs
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