Add py.typed marker

[clo-vis]

It seems MIP is fully typed. It does not include, however, the py.typed marker file, which is required to be PEP 561-compliant, and allow users of the library to type-check their code against the type annotations.

[rschwarz]

I think that before this adds value, some of the annotations need fixing, eg replacing numbers.Real with float. We found that using mypy-generated stubs yield a lot of warnings (from mypy).