Add parameter $item to return_exhibit_pages_containing_current_item

This parameter is not required; the default value is NULL.
Adding the parameter was needed for the CulOmekaXmlOutput plugin,
which calls this method to get the item-in-context information.
When generating the cul-omeka-xml output within a collection, need
to pass the item to the method because there is no way of finding
it from the context of the collection, even though the method is
called for each item in the collection - we were getting an error for
the call get_current_record('item')