Tile_qa_plot: add VCCDSEC info and alert

[araichoor]

This PR adds in the tile-qa*png plot:

• report of the min(VCCDSEC) value over all {camera,petals} for each exposure;
• a "red alert" message if min(VCCDSEC) is below 72000s=20h; that value is stored in desispec/data/qa/qa-params.yaml;
• and also a log.warning() message is written in that case.

We can surely change that threshold=20h value, based on our experience with those events: @schlafly ?

I picked that threshold=20h value based on this plot, which looks at the b0 campet VCCDSEC values for all exposures since Dec. 2021:

(file here: https://data.desi.lbl.gov/desi/users/raichoor/spectro/daily/vccdsec-b0-202112-202310.asc)

Comments:

• this tile-qa plots starts to be quite busy...
  • I squeezed a bit the per-exposure infos reported on the top to add this new VCCDSEC column
  • I re-arrange a bit the legend display in the n(z) plot
  • the "red alert" message also can overlap a bit some other text; if need be a detailed log.warning() message is written
• the desispec.tile_qa_plot.get_expid_vccdsec() returns a -99 value if the VCCDSEC` keyword is not present in the frame header
• data team: I apologize for the very-not-elegant code here... but hopefully it should work!

Below are few examples:

tile-qa-21822-thru20230309.png: that night was discussed here: #2023

tile-qa-25220-thru20231028.png: that recent night also was problematic (desihub/desisurveyops#156)

tile-qa-1863-thru20231031.png: a tile with no problem

[schlafly]

Thanks Anand. These look great. I think it's fine if the text overlaps, etc.; the main thing is that we have the information and won't miss it. I probably would have picked 12 hours but 20 is fine; in practice this is just going to trigger us to be worried about VCCDSEC and flag things as unsure pending further investigation.

[araichoor]

thanks @schlafly for the feedback.

btw I ve just verified that for 20231102, the three tiles 24608, 6997, and 27603 prompt some warning messages for petal=9; e.g. for 6997:

WARNING:tile_qa_plot.py:1023:get_expid_vccdsec: For EXPID=203220, the following VCCDSEC values are below the 72000 threshold: b9=3158.6, r9=3195.5, z9=3150.3
WARNING:tile_qa_plot.py:1023:get_expid_vccdsec: For EXPID=203221, the following VCCDSEC values are below the 72000 threshold: b9=4664.6, r9=4701.6, z9=4656.4