Add functions to read in outputs generated by sourmash

[taylorreiter]

This PR adds three functions that will read some of the outputs generated by sourmash:

• The csv generated by sourmash gather
• The csv generated by sourmash compare
• The csv generated by sourmash taxonomy annotate

As much as possible, I tried to follow conventions of package development laid out in this guide: https://r-pkgs.org/

Some of the files are automatically generated or edited by devtools::document(), roxygen2, and usethis::use_this(). This includes

• DESCRIPTION
• NAMESPACE
• man/*RD
• R/utils-pipe.R

I've punted on functions for

• importing a signature
• The csv output by sourmash sig describe
• The csv generated by sourmash taxonomy metagenome
• All of the LCA commands
• The files output by sourmash search and sourmash prefetch.

I'll tackle these later, but the functions in this PR are the ones I need to tackle #3 and #2 so I'm starting here.

@ctb & @luizirber you might be interested in this PR.