NF: add get_unscaled compatibility routine

This allow us to get the unscaled data from an image avoiding the rather
awful read_img_data function that I want to deprecate.
RF: remove yields from test_mask

The test generator makes it more difficult to debug; remove.
BF: get image slope using raw header

I was using the header slope to calculate expected error.  The current
version of nibabel doesn't return the read slope from `get_slope_inter`
as I was assuming here.  Directly read the written header to get the
slope.
RF: use nibcompat module to get unscaled data

mask computation preferred unscaled data, but the routine in nibabel was
broken from recent commits I had done on nibabel, and in any case was
not well designed.  Use a compatibility routine to use the new method in
nibabel if available.
RF: use nibcompat routines for affines, headers

Use nibabel compatibility routines for getting affines and headers from
nibabel images. This avoids running into future warnings when we
deprecate the get_affine, get_header methods.