'In' operator works on hdulist (issue #3060)

I added a __contains__ method to the Hdulist class, so that typing things like "a" in hdulist works. I implemented it by just doing a try/except with the index_of method, so it works with tuples of the form (name, extver) as well. 

Here is the original issue:
https://github.com/astropy/astropy/issues/3060
To be more specific, I don't want this line in the docstring at all; really this isn't a docstring users are ever likely to see anyways, but that `__contains__` implements the `in` operator is implicit.