Add functions to NAMESPACE using roxygen2 tags

[judith-bourque]

Issue

There has been version conflicts in the namespace file reported by @p2xcode.

Proposed solution

To facilitate maintenance of the package, please add @export roxygen2 tags to functions that should be listed as exports in NAMESPACE.

Namespace should only be edited automatically by roxygen2 when running devtools::document(). Running devtools::document() will override any manual edits.