Add static typing checks (mypy) and test in Travis CI job

[redeboer]

Currently, the types are only as hints to the user and for the API (https://github.com/ComPWA/tensorwaves/pull/27) — there are no checks on these. For better typing, it is better to use the typing module (from Python 3.5, so should be fine) along with mypy. This can also be added as a test to Travis CI.

_Originally posted by @redeboer in https://github.com/ComPWA/tensorwaves/pull/30/review_comment/create_

• [x] Add mypy to requirements and pre-commit https://github.com/ComPWA/tensorwaves/pull/48
• [x] Fix mypy errors in non-expertsystem modules
• [x] Add travis check on mypy https://github.com/ComPWA/tensorwaves/pull/48/commits/1f81304f80a47309f9082535075ca5ae08d10bdc

[redeboer]

In addition: Codacy is nice from a graphical point of view, but the linting checks there do not fully agree with the checks of pre-commit. For instance, all flake checks there are not performed.

On the other hand, if you would perform linting checks through Travis, there is no option anymore to temporarily ignore certain checks (as there is on Codacy).