Add NFS/NTS interface to desisurvey scheduler

[schlafly]

Adds Next Tile Selector functionality to the desisurvey scheduler.

The key addition of this routine to desisurvey is the NTS.py module that connects to the DOS NFS.py module to give the online system access to the desisurvey scheduler.

A few minor changes were made outside NTS.py:

• Planner and Scheduler save(...) methods were made atomic
• scheduler.next_tile(...) now accepts an program argument. Tiles are selected from this program.
• next_tile also takes a skylevel argument, which is presently ignored.
• tiles.index(...) now accepts a default False return_mask argument. If True, instead of raising an exception if an invalid tile ID is sent, the function returns a mask, indicating which IDs were valid.

[schlafly]

These changes should resolve the ra/dec <-> dec/ra mixup and improved the NTS documentation.

[dkirkby]

The changes look good, thanks! I am restarting the travis job that failed since it looks like a network glitch.

[dkirkby]

Tests passing now.