Improve lazy asset loading: use tarred files and checksums

[sibocw]

Clean up asset lazy loading module which is currently a code smell:

1. remote asset files on s3 are always tarred and paired with an sha256 checksum (.tar and .checksum)
2. flygym downloads the tar file into a temp dir and verifies data integrity via checksum
3. flygym decompresses the tarball into a folder
4. flygym calls pathlib.Path(temp_folder_path),replace(target_cache_dir), which is atomic and raises an error if some other thread has already created the target dir in the meantime (in which case it discards the temp dir holding the downloaded data)
5. added a script to make the tar and checksum files.