Refactor: clean up typing

[machow]

Let's add return types to things like render concretes, and ensure the dev experience is using types (to the degree possible, given that we're using plum for multiple dispatch).

Related:

• render_annotation needs documentation on website #100
• Documentation for render methods need to list input types and output types #99
• render methods should have return annotations #98

[r-leyshon]

Hi @machow not sure if this is the most appropriate issue to attach this bug we've found. Do you have any advice?
datasciencecampus/transport-network-performance#280

[machow]

Shoot--thanks for flagging I've opened issue #344 to track, and am opening a PR now!