Improve type annotations

[bwohlberg]

Current type annotation usage is in need of some attention:

1. Many functions and methods still do not have type annotations, e.g. jvp, jhvp, and vjp in scico.operator.
2. Figure out how to annotate a dynamically generated module so that the typing system is aware of the dynamically constructed functions.
3. Revisit the numerous lines of code marked as # type: ignore to determine whether the error that led to its insertion can instead be resolved by improved type annotation or use of type guards.
4. Review class structure with a view to minimization violations of the Liskov substitution principle (e.g. see issues with prox method).