Update the data model for Main Survey MTL

[geordie666]

This PR tweaks the data model and directory structures for MTL files related to the Main Survey. It:

• Fixes a bug to allow all types of MTL ledgers to be read, including SV3 and Main Survey 1.0.0 ledgers.
  • This fix, which addresses #749, is also included in #753, but I needed it to make progress in this PR, too.
• Changes the location of the "ZTILE" file (i.e. the file that includes zdone=True/False decisions) for the Main Survey:
  • For iterations of SV, this file was located at $ZCAT_DIR/tiles.csv
  • For the Main Survey, it will be located at $MTL_DIR/../ops/tiles-specstatus.ecsv (i.e. in the ops directory that is parallel to the mtl directory in the surveyops svn trunk).
• Includes a workaround to process SV3 files that have fibermaps corresponding to the Main Survey.
  • PLATE_RA and PLATE_DEC were added to fibermaps when we started observing main tiles, but during the sv3->main transition, some zbest- files in sv3 also included these added columns.

I will need to test this PR once Stephen and Julien have the tiles-specstatus.ecsv file in place in svn, so I'm leaving it as a Work In Progress for now.

[coveralls]

Coverage decreased (-0.09%) to 58.667% when pulling bdba75f88f4f50c00eb90383ba97b6132bf7c18c on ADM-new-MTL-model into f39455769628b7982fc18c1e2668a6d1161a3e87 on master.