Issue 5: convert dates to ISO-8601 format

[wdduncan]

See #5

[dwinston]

This still needs code added, right? Or is the PR intended to simply be an addition of example json files?

[wdduncan]

@dwinston I've added the code to change the add_date and mod_date fields to iso-8601 format. I've also added the nmdc_database.json.zip that is the output of the make run-etl target. I wasn't sure if needed to re-run make run-elt to incorporate the new data or if you could slurp in the zip file.