Add mapping_tool_id slot to model

[matentzn]

Resolves #449

• 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
• make sure any new slot is annotated with the appropriate added_in annotation
• run SSSOM-Py test suite against the updated model

This PR is adding mapping_tool_id as an entity reference to the SSSOM model. I have also included a note that if possible, this slot should be preferred over the less formal mapping_tool slot.

cc @nichtich @gouttegd

[nichtich]

Looks good. Nitpicking comments in the issue #449.

[matentzn]

@nichtich I added you to the SSSOM core team so your reviews can count;

For the rest, I will merge this PR as is anytime after Wednesday 25th.