BF(DOC): there is no .save in nibabel's Nifti1Image -- to_filename to be used
BF+TST: compatibility with numpy 1.7.1 -- use is_the_same_base helper more
BF: do not rely on identical FP results in a test_over_axis

since numpy 1.7.0, for performance improvements, order of summation
could be different thus affecting precise FP result value.
See
http://permalink.gmane.org/gmane.comp.python.numeric.general/54061
for more discussion/examples