Package TWiki::AggregateIterator 
combine multiple iterators
 new(\@list, $unique) 
Create a new iterator over the given list of iterators. The list is not damaged in
any way.
if $unique is set, we try to not repeat values.
Warning: $unique assumes that the values are strings (so works for cUID's )
 hasNext() -> $boolean 
Returns false when the iterator is exhausted.
my $it = new TWiki::ListIterator(\@list);
while ($it->hasNext()) {
   ...
 next() -> $data 
Return the next entry in the list.
The iterator object can be customised to pre- and post-process entries from
the list before returning them. This is done by setting two fields in the
iterator object:
 
-  {filter}can be defined to be a sub that filters each entry. The entry     will be ignored (next() will not return it) if the filter returns false.
-  {process}can be defined to be a sub to process each entry before it     is returned by next. The value returned from next is the value returned     by the process function.