Add Type Hinting

[cflynn3]

Currently importing RESQPY into a python file in another project and running the Mypy type checker generates a series of errors: error: Skipping analyzing "resqpy": module is installed, but missing library stubs or py.typed marker [import]. This is because Mypy cannot detect the types present in RESQPY due to a lack of type hints.

[andy-beer]

I'm not sure we should regard this as a bug. We considered using type hints quite a while back and concluded that the value they would add did not justify the time and effort to add them throughout resqpy. I'm therefore going to relabel this as an enhancement.