Add type hints to eigen.py

[juliahou]

Hi! I'm new to open source projects, and I thought this project would be a great place to get started. I'd love any feedback/hints/corrections/advice, especially as I'm not entirely sure I did this correctly.

[marcharper]

Hi @juliahou8 ! Thanks for the contribution. I have one comment: rather than a namedtuple type I think it would be simpler to just use a tuple type (also from the typing module) instead of the Eigenvector namedtuple type:

Tuple[numpy.ndarray, float]

This is because we're not actually using a namedtuple (which comes from collections).

Otherwise it looks good to me!

[marcharper]

Thanks! We have a two review policy for PRs so we'll need a 👍 from one of the other core devs. Usually this is pretty quick (but one is travelling so it may be a little longer than usual).