add downsample_axis code from image_tools.  Definitely need a test, since I don't really know what's going on with WCS yet
add downsample_axis code from image_tools.  Definitely need a test,
since I don't really know what's going on with WCS yet

jzuhone/spectral-cube,keflavich/spectral-cube,e-koch/spectral-cube,low-sky/spectral-cube,radio-astro-tools/spectral-cube