on gain, dl, and sensor type change, chans drop response.

nexus/nexus.py

@@ -284,6 +284,7 @@ class StationNode(InventoryNode):
             except AttributeError:
                 chan.data_logger = Equipment()
                 chan.data_logger.gain = value
+            chan.response = None
         return True
     dl_gain = property(get_dl_gain, set_dl_gain)
     FM.append(dl_gain)
@@ -303,6 +304,7 @@ class StationNode(InventoryNode):
                 chan.data_logger.type = DLS.name(value)
             except AttributeError:
                 chan.data_logger = Equipment(type=DLS.name(value))
+            chan.response = None
         return True
     dl_type = property(get_dl_type, set_dl_type)
     FM.append(dl_type)
@@ -342,6 +344,7 @@ class StationNode(InventoryNode):
                 chan.sensor.type = SENSORS.name(value)
             except AttributeError:
                 chan.sensor = Equipment(type=SENSORS.name(value))
+            chan.response = None
         return True
     sensor_type = property(get_sensor_type, set_sensor_type)
     FM.append(sensor_type)