XimeaCamera._fetch_data (fixup 9fe1030)

Fix two issues introduced when removing code duplication:

1. When doing software trigger return None if we are acquiring but
have not triggered yet otherwise it falls through to the error about
unknown trigger type.  This partially fixes issue python-microscope#131.

2. return the acquire data when doing non-software trigger (fix
indentation level of the return data statement)

microscope/cameras/ximea.py

@@ -44,8 +44,10 @@ def _fetch_data(self):
             return
 
         trigger_type = self._handle.get_trigger_source()
-        if trigger_type == 'XI_TRG_SOFTWARE' and not self._triggered:
-            return
+        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)
 
@@ -57,7 +59,7 @@ def _fetch_data(self):
             _logger.info('Sending image')
             if trigger_type == 'XI_TRG_SOFTWARE':
                 self._triggered = False
-                return self.img.get_image_data_numpy()
+            return self.img.get_image_data_numpy()
         except Exception as err:
             if getattr(err, 'status', None) == 10:
                 # This is a Timeout error