Defer 'expensive' imports until they are needed.

[rwiker]

Also, do a little cleanup based on flake8 findings.

Issue

N/A

Short description

Move some "expensive" imports from top-level of files into the methods where they are needed.

Also (incidentally/accidentally): added a new example notebook showing performance gains when using split tables for aggregation.

Pre-review checklist

• Read through all changes. No redundant print() statements, commented-out code, or other remnants from the development. 👀
• Make sure tests pass after every commit. ✅
• New/refactored code is following same conventions as the rest of the code base. 🧬
• New/refactored code is tested. ⚙
• Documentation has been updated 🧾

Pre-merge checklist

• Commit history is consistent and clean. 👌

[ovejorgensen]

LGTM