Added function cul_return_exhibit_pages_containing_current_item
Added/Modified code for the link back to exhibit page

Moved a lot of the code that handles the link back to the
exhibit page(s) containing an item from item.php into
functions in CulCustomize. Also moved these functions into
the class definition, and made them static.

As far as the modifications are concerned, the link text now
contains the title of the exhibit page, as well as the parent
page. Also, added code to prevent duplicate printing of a link,
as well as short-circuiting code in case we arrived at the item
page without going through an exhibit page.
Changed word "exhibit" to "exhibition" in heading

For the heading for the list of links to other exhibit pages
containing the item, changed the word "exhibit" to "exhibition"
Test if need "also", and return if no exhibit pages

The code now tests if the word "also" is needed in the heading
to the links to exhibit page(s) - word is not needed if there is not a
link back to the exhibit page that lead the user to the item page.
Also, added a test that just returns if there are no exhibit pages
to link to.
Remove debugging code