obs_sdss should use pydl.yanny instead of it's own copy thereof

Inside obs_sdss there is a yanny.py that looks like it was copied from either sdss_python_module or pydl. We should just depend on pydl (https://github.com/weaverba137/pydl), so we can use whatever improvements it gets for free, and to prevent yet annother yanny reader floating around.