Update Application linking to user specific /Applications folder

@paulmc @mwebster , on macOS the post_install script currently links some applications to /Applications even if the user has requested to install FSL in a non-root location (in the case of non-admin users). I noticed this when installing FSL on the development M1 mac mini we have. I'm not an admin on that machine. /Applications is only writable by users with admin privs on macOS.

We could install to $USER/Applications in the case of non-root/admin installations. The application links would still work for that user, and FSLeyes.app etc. would still show up in the usual locations such as the macOS launchpad.

Opening the issue to start the discussion of the change, and can implement later if we agree on it.