Type Checking

[eitanturok]

What does this PR do?

Add type checking.

What issue(s) does this change relate to?

Before submitting

• Have you read the contributor guidelines?
• Is this change a documentation change or typo fix? If so, skip the rest of this checklist.
• Was this change discussed/approved in a GitHub issue first? It is much more likely to be merged if so.
• Did you update any related docs and document your change?
• Did you update any related tests and add any new tests related to your change? (see testing)
• Did you run the tests locally to make sure they pass?
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[eitanturok]

Ready for a review. There are still ~300 type checking errors. I'm going to address those in a follow up PR.