Implement a fix for the function install() so that it works on Windows OS

As described in #43 and #44 the current implementation of `install()` places the relevant csv files on the desktop rather than in the user's working directory as is expected. The solution to this problem was to change the default value of the `data_dir` argument from NULL to "." as well as specify that the `retriever` CLI always use the path arguments rather than only using them optionally when `data_dir` was not NULL.