Type annotations for PyLaGriT

[ongchi]

Hi there!

I've been using PyLaGriT for a while couple weeks, and I've noticed that it could benefit from the addition of type annotations. Python 3.5 introduced type hinting, and since then, it has become a widely adopted practice in the Python community.

It's really helpful because many potential errors could be hinted before runtime, and this feature is not only for IDE but any modern code editor would supported. If this project would receive external PRs (BTW, I believe the CLA site is down), I'm also happy to contribute.