Refs #8528. Correct error related to reduction in batch

Because assignSample cleaned the slices definition, it lacked support for
slices. Now, it is cleaned on loading mask.

Code/Mantid/scripts/SANS/ISISCommandInterface.py

@@ -153,6 +153,10 @@ def MaskFile(file_name):
         @param file_name: the settings file
     """
     _printMessage('#Opening "'+file_name+'"')
+
+    # ensure that no slice string is kept from previous executions.
+    ReductionSingleton().setSlicesLimits("")
+
     ReductionSingleton().user_settings = isis_reduction_steps.UserFile(
         file_name)
     status = ReductionSingleton().user_settings.execute(

Code/Mantid/scripts/SANS/isis_reducer.py

@@ -174,16 +174,17 @@ def _init_steps(self):
         self._rem_nans =      sans_reduction_steps.StripEndNans()
 
         self.set_Q_output_type(self.to_Q.output_type)
+        # keep information about event slicing
+        self._slices_def = []
+        self._slice_index = 0
+
 
     def _clean_loaded_data(self):
         self._sample_run = Sample()
         self._can_run = Can()
         self.samp_trans_load = None
         self.can_trans_load = None
         self.event2hist = isis_reduction_steps.SliceEvent()
-        # keep information about event slicing
-        self._slices_def = []
-        self._slice_index = 0
 
 
     def __init__(self):