add force flag to savetohdf

by default false. if true overwrite existing group

ess/TraceWin.py

@@ -740,15 +740,21 @@ class density_file:
             self.Np=self._sum_merge(o,'Np')
             self.Nrun+=o.Nrun
 
-    def savetohdf(self,filename='Density.h5',group='TraceWin'):
+    def savetohdf(self,filename='Density.h5',group='TraceWin', force=False):
         '''
         Saves data to HDF5
         '''
-        import h5py
+        import h5py,sys
 
         fout = h5py.File(filename,'a')
         if group in fout:
-            del fout[group]
+            if force:
+                del fout[group]
+            else:
+                if sys.flags.debug:
+                    print("Group {} already exist in {}".format(group,filename))
+                return
+
         group = fout.create_group(group)
 
         # header attributes..