Overwrite *.dylib on macOS

[eslickj]

If you try to run get-extensions on macOS, and you've run it before, you'll get a permission error. Seems macOS doesn't really like you to delete or overwrite dylib files. The workaround for now is just to delete the files in the $HOME/.idaes/bin directory before getting extensions.

[ksbeattie]

Perhaps add a --force to overwrite the *.dylib files if they exist (and let the user know about that option if it fails when the files are there). Probably do not want to remove the bin dir automatically!