Incorrect warning when processing Session with missing Antenna FITS files

[tchamberlin]

In cases where no Antenna files are produced, SDFITS appears to fill 0 as the ELEVATIO. This causes Dysh to report a warning that all scans have an elevation below 5 degrees, which is impossible.

To reproduce:

from pathlib import Path
from dysh.util.download import from_url
from dysh.fits.gbtfitsload import GBTFITSLoad
TGBT24B_612_09 = GBTFITSLoad("/home/sdfits/TGBT24B_612_09/TGBT24B_612_09.raw.vegas")

That will produce this warning:

/home/dysh/release/0.4/dysh/src/dysh/fits/gbtfitsload.py:2307: UserWarning: Scan(s) 2,4,5 have an elevation below 5 degrees. The GBT does not go that low. Any operations that rely on the sky coordinates are likely to be inaccurate (e.g., switching velocity frames).
  warnings.warn(warning_msg(",".join(low_el_scans), "an", "elevation", "5 degrees"))

Note that the raw data for this Session do not include any Antenna files:

$ ll /home/gbtdata/TGBT24B_612_09
total 212K
drwxrwxr-x    7 monctrl monctrl 4.0K Oct 16 08:35 ./
drwxr-xr-x 4638 monctrl monctrl 172K Oct 16 13:46 ../
drwxrwxr-x    2 monctrl monctrl 4.0K Oct 16 08:55 GO/
drwxrwxr-x    2 monctrl monctrl 4.0K Oct 16 08:55 IF/
drwxrwxr-x    2 monctrl monctrl 4.0K Oct 16 08:55 LO1A/
drwxrwxr-x    2 monctrl monctrl 4.0K Oct 16 08:55 LO1B/
drwxrwxr-x    2 monctrl monctrl 4.0K Oct 16 08:33 Rcvr1_2/
-rw-rw-r--    1 monctrl monctrl  12K Oct 16 08:56 ScanLog.fits

This is because the telescope was stowed, and the Antenna was removed from the ScanCoordinator for these scans. This will very commonly be the case with test data.

I'm not sure what the correct behavior here would be. Arguably Dysh is doing the correct thing already. Alternatively, it could have a special case for elevation of 0.0.

But I figured it was at least worth discussing