ENH: add flag in rtkosem application to store the normalization images

Add flag to rtkosem application to choose to store or not the
normalization images. Storing the normalization images can increase the
speed of the reconstruction but could also create memory issues if the
size of the normalization images is too big.

applications/rtkosem/rtkosem.cxx

@@ -90,7 +90,7 @@ main(int argc, char * argv[])
 
   osem->SetNumberOfIterations(args_info.niterations_arg);
   osem->SetNumberOfProjectionsPerSubset(args_info.nprojpersubset_arg);
-
+  osem->SetStoreNormalizationImages(!args_info.nostorenormalizationimages_flag);
   REPORT_ITERATIONS(osem, rtk::OSEMConeBeamReconstructionFilter<OutputImageType>, OutputImageType)
 
   // Write

applications/rtkosem/rtkosem.ggo

@@ -9,3 +9,4 @@ option "niterations" n "Number of iterations"                                  i
 option "input"     i "Input volume"              string          no
 option "nprojpersubset" - "Number of projections processed between each update of the reconstructed volume (several for OSEM, all for MLEM)" int no default="1"
 option "betaregularization" - "Hyperparameter for the regularization"          float  no   default="0"
+option "nostorenormalizationimages"       - "Do not store the normalization images during the reconstruction"   flag   off