Static typing fixes

[frazane]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've formatted the new code by running poetry run pre-commit run --all-files --show-diff-on-failure before committing.
• I've added tests for new code.
• I've added docstrings for the new code.

Description

I added two small improvements to the static typing which will make coding with GPJax a bit easier, especially in fully featured IDEs.

• construct_posterior and __mul__ are overloaded such that the correct posterior (abstract, conjugate or nonconjugate) is returned when we construct the posterior (current behavior is that an AbstractPosterior is always returned). This way we get proper tab-completion and information (signatures, docstrings) when mouse-hovering on an object.
• pass (and return) a TypeVar (with Module as the upper bound) in fit instead of Module itself, such that we retain the right static typing information about the model we are using