Add mypy to CI job and type hints for one class.

[eliotwrobson]

Changes to test type hints and integrating a typechecker into the CI job. This was to test feasibility of adding type annotations to the package (per the issue). This was surprisingly easy to do, I basically just copied the types listed in the docstring.

Relevant issues/PRs:

https://github.com/fatiando/pooch/issues/330

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