Enable use of scheduled hour angle in fba on the fly

[sybenzvi]

PR to address #174. The fba-main-onthefly.sh script can take HA as an argument (calculated externally by NTS) and use it rather than the DESIGN_HA in tiles-main.ecsv.

[coveralls]

coverage: 30.377% (-0.06%) from 30.435%
when pulling d43852f on use_sched_hourangle
into e4b2ac0 on main.

[sybenzvi]

@schlafly, I could use a sanity check of this PR. The script fba-main-onthefly.sh now handles optional arguments and I altered NTS.py to always pass the current HA of a tile to the script as the scheduled HA. I tried to it with minimal alterations to the existing code.

The change is not backwards compatible with the past ~5 years of fba on the fly because I'm forcing the use of a scheduled HA in the next_tile function. Before we merge this, it's probably a good idea to enable the old behavior by default, with the new behavior trigged by an option passed to next_tile. However, if you think the logic is otherwise OK, that won't be hard to implement.

[schlafly]

That all looks right to me. Too bad that there's not a nicer hour angle interface in desisurvey, but I think what you've done is right. There is no need for compatibility backwards in time with the NTS since we do not do multiple realizations of the survey strategy. I think the logic is good and in your place I would have made it always-on as you have done, without an option for turning it off.