Use mypy type checking

[derNarr]

mypy is a statical type checker for python. It needs additional annotations into the source code and we should use it.

http://mypy-lang.org/

[dekuenstle]

When using the type annotations we can only support python >= 3.5 https://www.python.org/dev/peps/pep-0484/ .

Still static type checks are worth it imo.

[dekuenstle]

This issue is not as easy as thought:
For the used libraries we require stub files containing the api's types.
I have problems finding them.

• integration into tox

Required stubs:

• numpy: https://github.com/machinalis/mypy-data
• pandas
• xarray
• ctypes
• matplotlib

Still we could ignore this at the moment by --ignore-missing-imports flag.

[dekuenstle]

numpy began writing their official (but experimental) stubs: https://github.com/numpy/numpy-stubs