better backup solution for DESI_SPECTRO_DARK

[sybenzvi]

Since October 2024, we've been using a daily wget to synchronize the DESI_SPECTRO_DARK folder at NERSC with the DESI cluster at KPNO. This replaced a previous rsync from perlmutter. I'm told that support for data transfers via wget will not continue indefinitely, but access to SVN repos will be allowed.

I recall that DESI_SPECTRO_DARK was split off from the DESI_SPECTRO_CALIB repo because the dataset was getting too large. The current set of bias and dark files in DESI_SPECTRO_DARK uses about 120G on disk, with ~40G in the bias frames and ~80G in the dark frames, compared to about 60G in DESI_SPECTRO_CALIB.

If we need to keep the repo below a certain size, we might sort them into separate repos by spectrograph or by year. For the convenience of maintainers (@Waelthus and @julienguy) we could probably still approximate the appearance of a single DESI_SPECTRO_DARK folder using svn:externals. To handle the changes to the folder structure, some work would need to be done to the CalibFinder code.