fix incorrect QSO selection test about desitarget

[Cyeche]

Summary: desitarget.test.test_cuts function test_single_cuts is incorrectly testing that QSO target selection does not depend upon deltaChi2 nor wise_snr. Fix that.

Background: originally cuts.isQSO allowed primary and objtype (psftype) to be optional parameters as a minor convenience for template generation so that it could focus on the fluxes and not have to also pass in arrays of primary=True and objtype='PSF'. A test confirmed that the selection would work with or without them.

Current problem: Since then, cuts.isQSO gained two more optional parameters deltaChi2 and wise_snr. The tests were expanded to include those as optional, but incorrectly is testing that they don't make any difference:

    qso1 = cuts.isQSO(gflux=gflux, rflux=rflux, zflux=zflux, w1flux=w1flux, w2flux=w2flux,
                      objtype=psftype, primary=primary, deltaChi2=deltaChi2, wise_snr=wise_snr)
    qso2 = cuts.isQSO(gflux=gflux, rflux=rflux, zflux=zflux, w1flux=w1flux, w2flux=w2flux,
                      objtype=None, primary=None, deltaChi2=None, wise_snr=None)
    self.assertTrue(np.all(qso1==qso2))

As the test the tests on deltaChi2 and wise_snr are not mandatory and as we just want to see the effect of requiring PSF type object, I would be in favor of just testing PSF (we can remove the primary object test because primary seems to be deprecated…) Therefore we just need:

qso2 = cuts.isQSO(gflux=gflux, rflux=rflux, zflux=zflux, w1flux=w1flux, w2flux=w2flux,
objtype=None, primary=primary, deltaChi2=deltaChi2, wise_snr=wise_snr)

This accidentally passes for some but not all of the test sweep files in test/t. It turns out that our tests use os.walk under the hood to grab the first sweep file it finds, and which file gets selected depends upon where the code is installed. I accidentally tripped over this yesterday when I found that the identical code installed on edison $SCRATCH failed tests while it worked when installed under $HOME... :(

Also note that cuts.isQSO uses wise_snr but cuts.isQSO_randomforest does not.

wise_snr is not used with random forest algorithm…

Christophe.

[moustakas]

The templates aren't properly using these cuts anyway since the QSO templates don't extend into the mid-IR (see desihub/desisim#152), but it seems like this issue could be resolved if we segregated the color-cuts (whether the "pure" color-cuts or those based on the Random Forest) from the other catalog-level cuts on morphology (aka deltaChi2) and wise_snr.

For example, the templates code could call an isQSO_colors() function whereas isQSO() could take the output Boolean array from isQSO_colors() and apply any additional catalog-level cuts. Doing this refactor (@Cyeche?) and fixing the tests should then be trivial. Thoughts?