Add curie_map element to data model

[matentzn]

Resolves #225

• docs/ have been added/updated if necessary
• make test has been run locally
• tests have been added/updated (if applicable)
• CHANGELOG.md has been updated.

If you are proposing a change to the SSSOM metadata model, you must

• provide a full, working and valid example in examples/
• provide a link to the related GitHub issue in the see_also field of the linkml model
• provide a link to a valid example in the see_also field of the linkml model
• Update the spec to reflect the fact that the curie_map is now part of the model

This PR adds the curie_map as an official property to the SSSOM data model.

For testing this, I followed @gouttegd great docs. But I also ran some of my own, local tests to ensure that sssom-py does not break.

[matentzn]

Oops closing.. #376

Missed this.