Negative exptimes

[michaelJwilson]

Surveysim output contains negative exptimes. These look to be solely the last exposure in the night, but lead to further spurious ~10s exposures where the (negative) SNR2FRAC increments upwards on the same TILEID.

Confirmed with @changhoonhahn - both for Twilight=True. Twilight=False is untested as yet.

[dkirkby]

Twilight=True is not well tested, so I am not too surprised. Hopefully it is easy to locate the bug, knowing it depends on twilight=True, but I won't get a chance to look at this in the next few days so feel free to jump in.

[michaelJwilson]

Correction to above, the neg. exposure times look to be those that cross bright -> dark transition
(consistently have ~ -15. for sun alt. at exp. mjd). Looking at a fix now.

[michaelJwilson]

L145 here: https://github.com/desihub/surveysim/blob/master/py/surveysim/nightops.py

Twilight Bright program finishes between receiving next_tile and new_field setup finishing.

[michaelJwilson]

Created pull request #67