add nucl mappings, slight reorg

[bluegenes]

Many thanks to contributing to czbiohub/sencha!

Please fill in the appropriate checklist below (delete whatever is not relevant). These are the most common things requested on pull requests (PRs).

PR checklist

• This comment contains a description of changes (with reason)
• If you've fixed a bug or added code that should be tested, add tests!
• Ensure the test suite passes with pytest . (command to run: pytest or make coverage if you want to see which lines don't have tests yet)
• Make sure your code is linted and autoformatted using black (black . --check).
• Documentation in usage.md is updated
• README.md is updated

[olgabot]

Thank you! Sorry to be nitpicky, but could you add some tests for the functions that are in this PR?