Add support and tests for BDS 6,1 emergency messages

ACAS RA messages (subtype 2) are not yet implemented. These messages
are the same format as BDS3,0 starting at byte 9

tests/test_adsb.py

@@ -80,6 +80,11 @@ def test_adsb_velocity():
     assert adsb.altitude_diff("8D485020994409940838175B284F") == 550
 
 
+def test_adsb_emergency():
+    assert not adsb.is_emergency('8DA2C1B6E112B600000000760759')
+    assert adsb.emergency_squawk('8DA2C1B6E112B600000000760759') == '6615'
+
+
 # def test_nic():
 #     assert adsb.nic('8D3C70A390AB11F55B8C57F65FE6') == 0
 #     assert adsb.nic('8DE1C9738A4A430B427D219C8225') == 1