Add test for files exported by H.E.S.S.

[joleroi]

This PR introduces
gammapy/irf/tests/test_hess.py

The idea is the following

• Produces reference files with gammapy-extra/test_datasets/reference/make_reference_files
• This also creates a YAML file that contain the datastore used to created the reference files
• test_hess.py read the IRF files from the same datastore that was used to create the reference files and compares the info() output to the reference files.
• That way all changes in the classes or the reference file immediately catch our eyes

The tests of the functionality of the IRF classes remain in the old test files, e.g. test_effective_area.py. But here only the classes themselves are tested irrespective of real files. Optimally these test would work on simulated IRFs where we know extact numbers

[joleroi]

FYI ignore changes to gammapy/irf/effective_area_table.py, These are the same as in #467. I continues that branch.

If we all agree on the general procedure I will expand this method to all IRFs

[cdeil]

Looks good to me.

There's this travis-ci fail though:
https://travis-ci.org/gammapy/gammapy/jobs/111526822#L1897

Can you reproduce and fix or should I have a look?

[joleroi]

You can have a look at
https://travis-ci.org/gammapy/gammapy/jobs/111685918#L443
if you have time

[cdeil]

That's a timeout that happened when installing the dependencies.

I've restarted build 2038 just now by clicking the restart symbol at the top right here:
https://travis-ci.org/gammapy/gammapy/builds/111685915

If the error is still there I can have a look on what changed on the conda side.

[joleroi]

I don't think this is merely a timeout, after restarting the test it again fails when trying to update setuptools. I will try to fix it ..

[cdeil]

But you can't reproduce the issue on your machine, right?

[joleroi]

Wait, maybe I screwed up restarting the tests properly ...

But anyway. Locally I don't have to download anything, right?

[joleroi]

Closing this now. I will update the other IRFs in a second PR next week