setBiocMirror sets all Bioconductor repositories
(software, data, annotation, etc.). so that they are
directly available to install.packages. It
differs from chooseBioCmirror in that it
effectively enables the repositories.
getBiocMirror is a shortcut for
getOption('BioC_mirror'), which returns the
current Bioconductor mirror as used by biocLite.
getBiocRepos returns urls to all Bioconductor
repositories on a given mirror.
setCRANMirror sets the preferred CRAN mirror.
CRAN simply contains the url of CRAN main mirror
(http://cran.r-project.org), and aims at
simplifying its use, e.g., in calls to
install.packages.