Improve 7zip download

Currently, 00_get_data.Rmd checks for the existence of 7zip, and if it is not present then it downloads it to a local bin/ directory and runs that. However, functions which are imported from packages may not know about this. This came up with hyRefactor::download_elev() and might come up elsewhere. Since hyRefactoris at EOL, we should change the existing 7zip download so that it puts the 7z.exe somewhere in the user's path rather than in that bin/ directory (users may not necessarily have ability to change their own path, but should have ownership of some directory or other in their path).

CC @mewieczo