tests for "big" fetch_*() functions

[orbeckst]

We should test all fetch_*() functions/datasets (see #6 and PR #16 ). The "big" datasets have sizes > ~150 MB and download takes a long(ish) time.

We should find a way to mock-out the download or replace it with something tiny. It would nevertheless be good to ascertain that the download URLs are still valid (e.g. by download the first few bytes).

[orbeckst]

@richardjgowers I tentatively assigned you as the King of Mocks. I don't think that this is super-urgent but I wanted an open issue as a reminder (as opposed to a discussion on closed #6).

[orbeckst]

This has been addressed by marking most of the dataset fetch tests online: test with

pytest -m online

to run them.

Perhaps we can make this a cron job.