'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
Just update this docstring and this should be fine.  Something more along the lines of what it actually tests.  For example
```
"""
Returns `True` if ``HDUList.index_of(item)`` succeeds.
"""
```