Make CoordColumn more reusable

by behaving like other Columns which explode per band by default.
RAColumn and DecColumn are now the non-exploding special cases
that still produce a single reference band column.
Change the behavior of intermediate CoordColumn preferred over
adding a second intermediate class in the class hierarchy.

python/lsst/pipe/tasks/functors.py

@@ -449,12 +449,9 @@ class FootprintNPix(Column):
 class CoordColumn(Column):
     """Base class for coordinate column, in degrees
     """
-    _allow_difference = False
     _radians = True
-    _defaultNoDup = True
 
-    def __init__(self, col, calculate=False, **kwargs):
-        self.calculate = calculate
+    def __init__(self, col, **kwargs):
         super().__init__(col, **kwargs)
 
     def _func(self, df):
@@ -468,6 +465,7 @@ class RAColumn(CoordColumn):
     """Right Ascension, in degrees
     """
     name = 'RA'
+    _defaultNoDup = True
 
     def __init__(self, **kwargs):
         super().__init__('coord_ra', **kwargs)
@@ -480,6 +478,7 @@ class DecColumn(CoordColumn):
     """Declination, in degrees
     """
     name = 'Dec'
+    _defaultNoDup = True
 
     def __init__(self, **kwargs):
         super().__init__('coord_dec', **kwargs)