switch some packages that are Imported (e.g. must be installed) to Suggests (can install and load the library if suggested packages aren't installed)

[taylorreiter]

• metacoder: only used for one function at the moment
• phyloseq: will only be needed to read taxonomy output to taxonomy table
• vegan: will only be used for rarefaction curves
• complexUpset/upsetr: will only be used for upset plots

I think I would like if only tidyverse packages are imports, and everything else is a suggest. TBD though :)