create_sframesky_pdf(): display IVAR=0 pixels in yellow

[araichoor]

This PR highlights IVAR=0 pixels in the sframesky-NIGHT.pdf plots in the the night qa.

I picked the yellow color, so that it does not attract too much the eye.

For instance for EXPID=154146 on 20221121, where b8 is flagged:

Current:

With this PR:

I am wondering if I should go for something a bit more fancier, with rather overplotting with transparency the yellow thing, so that one would still see the masked features (here b8), at the risk of losing in readibility...

any suggestion is welcome!

[araichoor]

mmhh some test fails, on something unrelated to this PR, I think:

FAILED py/desispec/test/test_linalg.py::TestLinalg::test_cholesky_invert - AssertionError: 1.2767227175480552e-07 != 0.0 within 7 places (1.2767227175480552e-07 difference)

[sbailey]

Thanks @araichoor . Overplotting with transparency to see the features that were masked could be neat but might take some time and tuning to get right, so let's take this improvement as-is and save further tuning for the future if we need it.