Mtltime timestamp

[araichoor]

This PR now defaults the args.mtltime fba_launch argument to the latest timestamp in the $DESI_SURVEYOPS/mtl-done-tiles.ecsv file for the considered args.program.
If no value is found, we default to the current time.

We currently add +1min to the MTL latest timestamp to account for current discrepancy between mtl-done-tiles.ecsv and the ledgers.
To be seen how to change that once the fix is done in desitarget

Besides, we did a black formatting of fba_launch.

[araichoor]

@geordie666 : Aside comment: with checking how the MTL ledgers reading cuts on the isodate, I see that it is now a "<" cut: https://github.com/desihub/desitarget/blob/b86ae8ebf61024e310e5d102723191d5f23d4007/py/desitarget/io.py#L2552-L2557
Would it be possible to make it a "<=" cut ?

[geordie666]

Would it be possible to make it a "<=" cut ?. It's not easy to switch this behavior and retain backwards-compatibility. I could add a kwarg specifying that the user wants <= rather than <. But, you could also just add a fraction of a second to the isodate that you pass.

[geordie666]

I'll add the "<=" kwarg when I add the LyA decisions for the full Main Survey MTL loop, though.

[araichoor]

thanks for adding a "<=" option in reading the mtl; that simplifies things on the fba_launch side (otherwise, I don t know if there corner case issues with adding a fraction of second to args.mtltime when reading the isodate0.

[geordie666]

I think this looks OK, now. I'm sure we'll find a few bugs as we work towards the new MTL loop, though!

[araichoor]

I pushed a last commit, triggered by a calm re-reading of the function.
I think it should be fine now:

>>> from fiberassign.fba_launch_io import get_program_latest_timestamp
>>> get_program_latest_timestamp("DARK")
'2021-05-12T19:16:48+00:00'
>>> get_program_latest_timestamp("BRIGHT")
'2021-05-12T19:16:38+00:00'
>>> get_program_latest_timestamp("BACKUP")
'2021-05-02T18:44:45+00:00'
>>> get_program_latest_timestamp("bla") is None
True

should we merge?