Homogenize naming of calibration flags

This implements RFC-498 and includes:
   - calib_psfUsed -> calib_psf_used
   - calib_psfCandidate -> calib_psf_candidate
   - calib_astrometryUsed -> calib_astrometry_used

to match the other exsiting flags:
   - calib_photometry_used
   - calib_photometry_reserved

and to match the recently updated documentation in afw/doc/table.dox, in
particular: "if the prefix of a field name represents a conceptual group
of multiple fields, use underscores to join the group name to the group
elements."

python/lsst/meas/astrom/astrometry.py

@@ -149,7 +149,7 @@ def __init__(self, refObjLoader, schema=None, **kwargs):
         RefMatchTask.__init__(self, refObjLoader, schema=schema, **kwargs)
 
         if schema is not None:
-            self.usedKey = schema.addField("calib_astrometryUsed", type="Flag",
+            self.usedKey = schema.addField("calib_astrometry_used", type="Flag",
                                            doc="set if source was used in astrometric calibration")
         else:
             self.usedKey = None

tests/test_astrometryTask.py

@@ -106,7 +106,7 @@ def testUsedFlag(self):
         # check that the used flag is set the right number of times
         count = 0
         for source in sourceCat:
-            if source.get('calib_astrometryUsed'):
+            if source.get('calib_astrometry_used'):
                 count += 1
         self.assertEqual(count, len(results.matches))