Update masking to be "all-sky" using Gaia/Tycho/URAT

[geordie666]

This PR is an extensive refactor of the masking procedures in desitarget. It includes:

• General functionality in desitarget to obtain and work with Tycho files.
• Deprecate using the sweeps to mask bright objects as this is now being done using the MASKBITS information from the imaging catalogs.
• Functionality to allow bright star masks to be constructed at different epochs by accounting for proper motions of Tycho/Gaia/URAT sources.
• Bright star masks are now explicitly written to a $MASK_DIR (which, together with a $TYCHO_DIR has been added to the etc/desitarget.module manifest).
• The radius-magnitude relationship is now a single, straightforward function.
• Refactoring of the test_brightmask unit tests to be simpler and have more coverage.
• Skies and supplemental skies are now always masked by default.
• A complete lack of backward compatibility (!), which I believe should be OK as the masking formalism wasn't yet being extensively used (while I waited on decisions on the imaging side).

This should mostly address #471. It could also be used to address #103.