BUG: When deleting a layer fallback to Projection=None only if there are no more images left in the layer stack.

BUG: When changing reference image, new spacing signs should always match the native spacing signs of the reference images to preserve reference image orientation

This reverts commit d25245e8.

This fix do not solve the issue with ignored and infinite values that affect all band of an image. This is why matrix type are used instead of vector for computation, in order to provide generic solution if number of sample per band is different.

This getter provide the ability to get the number of pixel with non infinity and not ignored values of an image.