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.
Merge pull request #245 from matthew-brett/refactor-to-new-api

MRG: refactor to affine, header properties

Use .affine, .header properties instead of .get_affine(), .get_header()
methods.