label tarballs with run ID and need to extract using --ignore-zeros

[tomkinsc]

During creation of incremental tarball chunks, each tarball can be created passing --label=value to GNU tar, where the label value includes additional information about the contents of the archive (helpful in cases where tarballs are renamed). We can use the label to store additional metadata such as the run ID (i.e. run folder name), and perhaps also a note that tarballs joined in a cloud bucket via gcloud storage objects compose (analogous to cat) must be extracted using GNU tar with --ignore-zeros specified.

See: https://www.gnu.org/software/tar/manual/html_node/label.html#label

[tomkinsc]

The label/note will not need to mention using --ignore-zeros once #8 is addressed (which obviates the need for skipping over runs of zeros in a concatenated tar stream).