Implementing type hints

[fgittins]

For one of own projects, I have written type stubs for dynesty in order to improve the static type checking of my code. I was wondering if there would be interest in adding official typing support to dynesty?

There are two options in providing type-hint support:

1. Include type stubs in the package itself (see NumPy for an example), or to add type hints to the Python scripts.
2. Similar to SciPy, create a separate repository with just the type stubs.

If you are open to the in-repo option, I would be happy to open a PR based on the stubs I have already written.