MRG: fix and deprecate read_img_data function

The read_img_data function was a hack to help Michael get the raw image
data out of Nifti images without applying the image scaling.  The new
API for consumable values in the header broke this code, but it was a
bad idea in the first place and it was in any case broken for MINC.

Add a 'get_unscaled' method for the ArrayProxy object to give a cleaner
API to get the unscaled data. Try and replicate the old behavior of
read_img_data but deprecate so we don't need to maintain it long term.