Add signing for macOS compiled files

[cjfullerton]

Sign all compiled files produced by pyinstaller so end user doesn't;t have to worry about this. Looks like it will be essential to making onefolder version work on macOS.

See this for an outline of something to try: https://localazy.com/blog/how-to-automatically-sign-macos-apps-using-github-actions

[cjfullerton]

Using the install script in this repo: https://github.com/hpcflow/install-folder-version and adding the suggested folder to path in .zshrc looks like it avoids the need for signing.

This can be the recommended macOS install method (rather than direct download) until this issue is resolved.