Set, check, and get prism download directorySource:
prism_set_dl_dir() sets the directory that downloaded prism data will be
saved to. The prism download directory is saved in the "prism.path" option.
prism_get_dl_dir() gets the folder that prism data will be saved to. It is
a wrapper around
getOption("prism.path") so the user does not have to
remember the option name.
prism_check_dl_dir() checks that prism download folder has been set. If it
has not been set, and in interactive mode, then prompt user to specify the
download location. If not in interactive mode, and it has not been set, then
set to "~/prismtmp".
path_check() is a deprecated version of