Use typing.Literal for boundary mode, padding mode, and orthogonalization mode

[cthoyt]

This PR does the following:

1. Creates a Literal type annotation for ptwt boundary modes
2. Uses this literal for type annotations. This has the benefit that it makes the type checker much smarter and also auto-generates docs
3. Re-write the docs for mode in many places to point to a centralized place where we can expand on padding.
4. Sets a lot of these parameters to be keyword-only. This will make code much more understandable in the future, and also allows for easier development/debugging

Did the same thing for convolutional boundaries as well as orthogonalization method.

[cthoyt]

@v0lta this is blocked by @v0lta since I want to isolate the formatting updates from the new black style