properly document dependencies, external functions, function exports

[atn38]

Alina ran into some trouble with dependencies on Monday. We should be more carefully document dependencies in a way that would prompt user machines to stall or auto-install packages we depend on upon installation or loading of BLEdatatools. Imo best practices would be:

1. Prefix any external functions with the double colon syntax, e.g. "readr::read_csv"
2. Document the packages the external functions come from in the DESCRIPTION file, e.g.

Imports: 
    readr

A quick way to programmatically edit the DESCRIPTION file is to use usethis::use_package("readr").
If the external package is not on CRAN but Github, e.g. bleutils, use usethis::use_dev_package("bleutils", remotes = "BLE-LTER/bleutils" and the output DESCRIPTION would read:

Imports: 
    bleutils (>= 0.1.0)
Remotes: 
    BLE-LTER/bleutils

3. Export BLEdatatools functions that we want to be user-facing (so, not the utility-functions) with the @export tag in the header, like so:

# cool useful metadata up here
#' @export

See commit 9939337 for an example of some cleaning up.