make apply_along_axis public and have it return a projection

cleanup
more cleanup, but also note that the previous commit renamed
"apply_along_axis" to "apply_function", which is more accurate
fix test: unnecessary comma added
fix return type from _apply_numpy_function.   Now that I think about it,
that should probably be made public too
make apply_numpy_function public and have sum, min, max return
projections by default

forgot an underscore

whitespace
make callspec of apply_function and apply_numpy_function same
added changelog entry
fix bug: this was introduced when branches were switched and data was
discarded, which I've never seen before
Merge branch 'apply_publicly' into my_supermaster

Conflicts:
	spectral_cube/spectral_cube.py