use xfail decorator on test_get_psm3

[wholmgren]

The ongoing failure of test_get_psm3 is conditioning us (or at least me*) to accept PRs with failing tests. We've been told NREL is going to fix their API, but it keeps not happening, so I really think we should go ahead and mark this test with pytest.xfail. Perhaps pytest.xfail(strict=True) so that the test suite alerts us to when NREL does fix it.

@CameronTStark can you fix this if you agree it makes sense?

• I wrongly approved change tools._scalar_out/_array_out arg name to avoid collision with builtin input function #800 and I've not been checking Implement IEC 61853 IAM calculations for diffuse irradiance #793 because I've been thinking that the test failures were only because of test_get_psm3.

[CameronTStark]

Will do @wholmgren

[mikofski]

So sorry all for any inconveniences or issues as a result of the continued test_get_psm3 failure!
Thanks @CameronTStark and @wholmgren for cleaning up after my PR puppy!