ENH: NaNCorrection and update of RD computation

-NaN correction: a Not-a-Number correction if performed on the AD, MD, RD and FA images once they are computed from the transformed DTI to avoid having NaN voxels that would create problem when computing regional statistics
-RD was computed by averaging lambda2 and lambda3 from dtiprocess. There is now a direct option to obtain RD from the DTI with dtiprocess. We updated the script to use this option.
BUG: 4-AverageCreation was not creating good averages anymore. It looked like images were not registered but just stack on top of each other. Modifying the value of gamma for each iteration seems to have corrected that issue.