bUnwarpJ: update stack methods to use modern calls.
bUnwarpJ: make javadoc comment more clear.
bUnwarpJ: improve javadoc comments.
bUnwarpJ: fix bug when using masks included in the input images.

Fixed bug reported by David Richmond that was making the plugin crash when using masks in the second slice of the input images. The problem was in the call to "setProcessor". It had to be adapted to the ImageProcessor type when the input image was a stack.