BUG : as image now have an unsigned spacing and a signed direction, we need to set spacing AND direction parameters if using itk filter, here we use the itk gradient filter

add Get and Set SignedSpacing in image and vector image otb class
modify spacing utilisation in GDALIO

BUG: Fixing bug #1467. No Update() should happen on filters whatsoever, unless you know for sure that the Udpate() call will not load whole image in memory. No local pointers to filters are allowed (because the streaming pipeline disappears at the end of the method)