Add optional typing stubs

[CFlaniganMide]

PEP 484 defines a standard to add type hints to files in a backwards compatible manner by adding stub files. MyPy fully supports this for their type checking, which is supported by many modern IDEs.

Adding type hints for everything would be fairly tedious, although ultimately helpful. Likely, the most value would be from defining types for datasets and eventlists/arrays, since these are accessed fairly regularly. Starting with ebmlite would also provide a lot of information implicitly, which would go a long way to helping this repo.