GtMultiVolView: reduced memory consumption of cell sum

Signed-off-by: Nicola Vigano <nicola.vigano@esrf.fr>

zUtil_TomoUtils/GtMultiVolView.m

@@ -102,7 +102,10 @@ classdef GtMultiVolView < GtVolView
         function vol = getMultiVol(vols, index)
             if (iscell(vols))
                 if (index == 0)
-                    vol = sum(cat(4, vols{:}), 4);
+                    vol = vols{1};
+                    for n = 2:numel(vols)
+                        vol = vol + vols{n};
+                    end
                 else
                     vol = vols{index};
                 end