Iterator for DBIDs.
 
 Important note: this iterator has a 
significantly different syntax
 and semantics than the Java iterators. It is much more aligned with C than
 with Java, but at the same time, the syntax is much more compatible with for
 loops.
 
 Usage example:
 
 
 for (DBIDIter iter = ids.iter(); iter.valid(); iter.advance()) {
   Object o = relation.get(iter); // Many interfaces allow direct use
   DBID id = DBIDUtil.deref(iter); // Materialize only if you need to!
 }
 
 
 
 We list some fundamental differences.
 
 iter.valid() refers to the current element,
 Iterator.next() to the next. 
 iter.advance() does not return an element. Use
 get... to access it. 
 DBIDIter.get... do not advance the iterator.