XimeaCamera._fetch_data: enable any hardware trigger mode.

Instead of error for any non-software trigger mode other than rising
edge, just continue since we don't really care.  We either have an
image waiting for us or we don't and trigger mode should not make a
difference on how we fetch the data.

Author: carandraug

microscope/cameras/ximea.py

@@ -44,12 +44,13 @@ def _fetch_data(self):
             return
 
         trigger_type = self._handle.get_trigger_source()
-        if trigger_type == 'XI_TRG_SOFTWARE':
-            if not self._triggered:
-                return
-            # else, continue to fetch an image
-        elif trigger_type != 'XI_TRG_EDGE_RISING':
-            raise Exception('unhandled trigger type %s' % trigger_type)
+        if trigger_type == 'XI_TRG_SOFTWARE' and not not self._triggered:
+            return
+        # else, we are either on 1) software trigger mode and have
+        # already triggered, in which case there should be an image
+        # waiting for us; or 2) any hardware trigger mode, in which
+        # case we try to fetch an image and either we get one or it
+        # times out if there is none.
 
         try:
             self._handle.get_image(self.img)