Consider pooch library for test data

[gerritholl]

Currently, to run the typhon tests completely, users need to checkout the typhon-testfiles repository to a specified path. If these test data are really needed for running the tests (and can't be mocked or created on the fly), the pooch library may provide a more git- and user-friendly alternative. Compared to the status quo this has the advantage that:

• Users only need a single package (typhon) to run the tests, and
• No data files would need to be stored within a git repository (for which git is not well suited).

[olemke]

Thanks for pointing out the pooch library. It looks really useful.
I've played around with it and added a first implementation for the typhon tests (only in my fork for now).
A script in typhon-testfiles creates registry files for pooch in the typhon/test directory which are needed to retrieve the files during the tests.
The get_testfiles_directory function will try to retrieve the testfiles via pooch if a local copy of typhon-testfiles is not available.