Some clean-up for the 1% Survey

[geordie666]

This PR implements some recent changes decided on Target Selection and Operations meetings, and cleans up some code related to MTL for the 1% Survey. It includes:

• Don't allow BGS_FAINT targets in DARK observing conditions.
• Trigger a warning when primary targets are encountered that might be too bright (total fluxes or Gaia magnitudes in any band brighter than 16th mag).
• Have a single function for calculating UTC time stamps.
• Functionality to read the state of a ledger strictly before a certain UTC (ISO) time-stamp.
• Refactor and speed up routines to match arrays on TARGETID.

[geordie666]

@araichoor and @ashleyjross: This PR includes your requested functionality to make UTC/ISO time stamps in a single function that can be shared by fiberassign scripts related to MTL. It also adds a keyword (isodate) to revert an MTL ledger to its state at a specific ISO time.

[coveralls]

Coverage increased (+0.04%) to 60.732% when pulling fe2db94 on ADM-gen-clean into e9d5ffe on master.