idiff and compare_Yee fixes

1. Lift the restriction that idiff would automatically fail two images whose pixel data windows aren't the same size.  Now, like oiiotool, when comparing two images it will consider the pixel range that is the union of the data windows of both images, and using the "black" wrap mode for Iterators will behave as if pixels beyond either image's data windows are comprised of 0 in all channels.  This fixes #189.

2. Modernize IBA::compare_Yee, which was the only remaining holdout within IBA that didn't take ROI and nthreads arguments and work in a modern way.  Much of its guts have been changed to use ImageBuf, Iterators, and IBA functions where applicable, instead of doing things on raw float buffers. In the process, we lift the restriction that the top level input images need to be float and need to be the same size.