Add typing/annotations

[aaronspring]

Helping

• pydata/xarray#5495
• https://github.com/pydata/xarray/issues?q=is%3Aissue+typing
• https://docs.python.org/3/library/typing.html
• https://realpython.com/python-type-checking/
• https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html
• https://github.com/Ouranosinc/xclim/blob/master/.mypy.ini