Update twilight definitions in targetmask YAML

[dkirkby]

The file desitarget/data/targetmask.yaml contains the following:

#- Observing conditions
#- These are a bitmask to allow target bits to specify multiple conditions
#- in which they are allowed to be observed.
#- See DESI-0311 and DESI-0333 for discussion of DARK vs. GRAY
obsconditions:
    - [DARK,        0, "Moon is down"]
    - [GRAY,        1, "Moon up; illum fraction<0.6 and (illum*elev)<30 deg"]
    - [BRIGHT,      2, "Moon up and bright (not GRAY)"]
    - [POOR,        3, "Very bad seeing, high extinction, or bright cloud cover"]
    - [TWILIGHT12,  4, "Between sunset/sunrise and 12 degree twilight"]
    - [TWILIGHT18,  5, "Between 12 and 18 degree twilight"]
    - [DAY,         6, "Daytime calibrations"]
    - [CLOSED,      7, "Nighttime but dome is closed due to rain, wind, dew..."]
    - [APOCALYPSE,  8, "Conditions are so bad that the world is ending anyway"]    

We are currently using 13deg and 15deg twilight in the afternoon planner (desisurvey.afternoonplan), so this issue is to review the twilight defns, and get everything in synch.

[geordie666]

@dkirkby: This seems like an easy issue to solve and I'm embarrassed that I've left it hanging for 3 years! Is desisurvey still using 13deg and 15deg twilight definitions? Should I update the definitions in the desitarget yaml file (which won't affect anything else in desitarget specifically, I think).