Prepare for CRAN submission

[x] run devtools::check() to get warnings and output that would make CRAN mad and fix accordingly
[x] 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)
[x] ensure tests are running properly and are as strong as they can be
[x] improve function documentation (e.g., rarefaction function)
[ ] add warning for unbalanced designs
[ ]

MoBiodiv / mobr