Use contentID to retrieve and cache data

[dfalster]

After meeting with @cboettig, we learnt about https://github.com/cboettig/contentid

This looks like a promising option for locally caching downloads. Plays nice with zenodo.

[fontikar]

@dfalster is this still relevant? We ditched this for APCalign. Can close for revisit later?

[dfalster]

We were waiting for contentID to support Zenodo. Seems to have this now, as of cboettig/contentid#95! (nice work @cboettig!)