Mcs 23jun

Documented and revised Cube functions, get_image(), get_start(), get_end(), get_rot(), median(). Removed Cube.rebin_median(), Image.add(), Image.add_poission_noise(), Spectrum.rebin_median(), Spectrum.median_filter(), test_image.py:test_add(), test_spectrum.py:test_rebin_median(). Modified DataArray.new_from_obj() to discard variances when var=False.