Add proper error message for hardware trigger

src/python_spectrometer/daq/atsaverage.py

@@ -56,10 +56,10 @@ class AlazarATS9xx0(DAQ):
             self.hardware_channel = string.ascii_uppercase[self.hardware_channel]
         if not isinstance(self.hardware_channel, Channel):  # noqa
             self.hardware_channel = getattr(Channel, self.hardware_channel)  # noqa
-        if isinstance(self.trigger_callback, str):
+        if self.trigger_callback == 'software':
             self.trigger_callback = self.card.forceTrigger
         else:
-            raise NotImplementedError('trigger_callback')
+            raise NotImplementedError('Hardware trigger not yet implemented. Please open a PR.')
 
         self.default_capture_clock_config = CaptureClockConfiguration(  # noqa
             CaptureClockType.internal_clock,  # noqa