Change CompositeDevice to reject invalid identifiers

Also updated StatusBoard and StatusZero to reject duplicate identifiers
(namedtuple doesn't pick 'em up because they're passed in a dict and
thus the dups are squashed prior to the call). Added tests for all the
relevant stuff.

tests/test_devices.py

@@ -91,7 +91,7 @@ def test_composite_device_sequence():
         assert len(device) == 2
         assert device[0].pin.number == 4
         assert device[1].pin.number == 5
-        assert device.namedtuple._fields == ('_0', '_1')
+        assert device.namedtuple._fields == ('device_0', 'device_1')
 
 def test_composite_device_values():
     with CompositeDevice(