Make OpenPNM codebase typed using mypy

[ma-sadeghi]

Not that it's even marginally urgent, just to put this idea out that might be useful for maintaining OpenPNM in the long run. mypy is an optional static type checker for Python. There are a bunch of benefits, but the one that stood out to me the most was that it makes the codebase much more robust and less reliant on proper unit testing.