Add type hints to signature.py

[rpgoldman]

Address #962

This is substantially complicated by the use of Pydantic.

/cc @thomasahle

[rpgoldman]

I could definitely use help here. I got started just fixing issue #962 , but tried to add more type hints, and have been having issues because mypy does not seem to understand that Signature() is a class-constructor, not an instance-constructor, and also because it is confused by its signature (inherited from BasicModel, I believe).