Create scripts for copying tarballs and restoring tarballs

[ChrisJChang]

When running safe_dl.sh, this PR now creates files with the download paths. The user can then call the copy_tarballs.sh script from the project directory, and it will copy all of these tarballs into a single folder, along with a file specifying the paths to copy them back out to.

If the user calls the restore_tarballs.sh script, it will put these into their correct locations.

The purpose is this is that it should make builds on HPC systems without internet easier, as the user can build locally, run the copy_tarballs.sh script, and then copy all required tarballs in one go. This should avoid a lot of careful copying individual files every time you nuke a build.

It does create quite a few downloaded_tarball_path.txt files, but this is because the safe_dl.sh script will be called from different locations depending on what is being downloaded.

This was created using claude, so it has automatically put itself as the author of these scripts in their comments at the top of the files.

This closes Issue #555

[anderkve]

Thanks, this looks like a useful and lightweight fix for #555.

But one suggestion (originally brought up by @ahye): Instead of having many downloaded_tarball_path.txt files being created at many different places, and then having the script search for them, would it perhap be better if the safe_dl.sh script gets as input the path to the main project directory, so it can simply record the tarball paths in a single downloaded_tarball_paths.txt file kept at some standard location?