Relax identifiers "format": "uri"

[agbeltran]

Currently, the identifier related schemas include "format": "uri" while the original intention was to support both strings (for accession numbers that are not URIs) and URIs when available.

The examples we have at https://github.com/datatagsuite/examples demonstrate this, as we supported strings as identifiers in addition to URIs, but the problem was that the validator wasn't catching the "format": "uri"restriction.

This issue was originated here: dcppc/crosscut-metadata#8 by @jonathancrabtree