Add type annotations

[dougiesquire]

We are now type checking xhistogram using mypy (#38). However, none of the code is static typed at this point.

It would be good to type annotate the existing methods.

[Badboy-16]

Hi, I would like to work on this issue and will submit a PR in the coming days :)

[dougiesquire]

Great @Badboy-16 , a PR would be very welcome!