DOC: remove refactoring old ideas, add to TODO
RF: replace get_affine() with affine property

We will soon deprecate the 'get_affine()' method on images; prepare for
that by removing all uses (but legacy testing) for 'get_affine()' on
images.
RF: change uses of get_header() to .header

We are going to deprecate the .get_header() method for images in favor
of the '.header' property.  Remove any uses of get_header() for
neatness.