Tile_qa_plot: get_expids_efftimes(): speed-up, with only reading spectra files

[araichoor]

[re-opening a PR requesting to merge in "daily" this time]

This PR addresses #1951.

It is a speed-up in the desispec.tile_qa_plot.get_expids_efftimes() function.
Instead of opening both all the spectra and the cframe files, this PR only opens the spectra files.

The output should be identical.
I verified this with running the function on ~100 random exposures of the himalayas redux.

For the problematic case (TILEID=80617 in himalayas), it reduces the computation time by 50% (or more, it depends).

[sbailey]

Thanks. The code looks good. I'll trust your reproducibility testing. The unreleated github unit test failures are due to some new incompatibility with scipy 1.10.x; in PR #1950 Anthony fixed this by pinning scipy==1.7.3 (same as NERSC). To avoid merge conflicts I won't do the same here, but I'll investigate a long term fix to support scipy 1.10.x (it might involve updating desimodel instead of desispec).