Add cph role to Authors@R field

[nanxstats]

From CRAN repository policies:

Where copyrights are held by an entity other than the package authors, this should preferably be indicated via cph roles in the Authors@R field, or using a Copyright field (if necessary referring to an inst/COPYRIGHTS file).

This PR adds the cph role (with the latest recommended copyright text) to the Authors@R field, to supersede the use of Copyright field.

A more practical benefit of using Authors@R is for the pkgdown site: a proper authors.html will be generated and linked as "More about authors..." under the Developers section of the sidebar on the landing page.