Update the MTL loop to work for secondary ledgers for sv3.

[geordie666]

This PR updates the end-to-end MTL loop for secondary ledgers, with a view to starting to use seconary ledgers during the 1% Survey. Features include:

• Adds a new "MTL tile file" called scnd-mtl-done-tiles.ecsv. This file can be used to check that updates to the secondary ledgers are keeping pace with updates to the primary ledgers, but doesn't control any survey behavior.
• Sets the default option in the run_mtl_loop script to running secondary ledgers whenever the primary ledgers are processed. This can be turned off by specifying --nosec.
• Catches some special cases for secondaries, such as making sure that secondary quasars have the same MTL behavior as primary quasars.
• Updates NUMOBS to more sensible numbers for secondary targets. For sv1 secondaries had a high NUMOBS so they would always be available. For sv3, the plan is to let their NUMOBS_MORE drop to zero and their PRIORITY drop to DONE.

[coveralls]

Coverage decreased (-0.09%) to 60.559% when pulling 2aaf36d13645bfe3ad721c031583ac408f90b495 on ADM-sec-MTL-sv3 into 8a7c1b0e192e737418deed6019e16e4cb50060b4 on master.