Add Mypy

[kinow]

When I use pyflow in a project to develop a workflow, I get a few warnings in my IDE that could be detected with Mypy. I am preparing a PR for the kwargs in families, suites, etc. But I thought perhaps a future enhancement could be to adopt Mypy? That could help detecting simple typing issues too, receiving contributions from others (i.e. enforcing that we are all aware of the types used), and also enable to someday use other linters or performance tools like mypyc.

For example of the warnings (see yellow-ish background colored text):

Thank you!