Don't double-compress clusters

[benmwebb]

Currently the all-models clusters are stored in .tar.gz files, even though each cluster is just a single file (a multi-model PDB). When deposited at Zenodo, these files are compressed again to make .zip files. This should be remedied so that each file is only compressed once, so that we can simply link to it from the mmCIF output and visualization software can easily read it.