WIP: expose dataobj, affine, header image properties

As threatened on the list, expose the image data container directly as
`dataobj`.  Move the code for caching the read data out of the image
proxy objects and into the image classes.

Expose `affine` and `header` properties, add docstrings for `get_affine`
and `get_header` to warn of pending deprecation.

@cindeem @fmorency @hanke @ksubramz - please scan these changes - I've touched all of analyze, ecat, parrrec and mghformat here.