Copy `file_md5_checksum` deprecated in xarray 0.18

[x] This PR addresses an already opened issue (for bug fixes / features)
- This PR fixes #155
[x] Tests for the changes have been added (for bug fixes / features)
[ ] Documentation has been added / updated (for bug fixes / features)
[ ] HISTORY.rst has been updated (with summary of main changes)
[ ] I have added my relevant user information to AUTHORS.md
What kind of change does this PR introduce?: Copies the file_md5_checksum function into utils.tutorial because it has been removed (without warnings) from xarray 0.18, which was released yesterday. In fact their whole "test dataset" retrieval code now makes use of pooch, instead of the manual in-house code that we copied here and in xclim. At xclim we decided it was not worth our time (for now) to also make this move: if it works don't fix it.

This copy is a minimal change that makes it work again.

Does this PR introduce a breaking change?: No
Other information: I would suggest to add a build against xarray@master, if it ain't too much trouble. It could help detect those issues earlier.

roocs / clisops