CRAN hotfix

Version 6.2.1 is a hotfix to address the failing automated CRAN checks for 6.2.0. Chiefly, in CRAN's Debian R-devel (2018-12-10) check platform, errors of the form "length > 1 in coercion to logical" occurred when either argument to && or || was not of length 1 (e.g. nzchar(letters) && length(letters)). In addition to fixing these errors, version 6.2.1 also removes a problematic link from the vignette.

For more information, please see the release notes of version 6.2.0.