PERF: use hashmap lookup instead of linear search.

Provides significant performance improvement in large scene loading task
(1200 models), from >20 minutes on master to 5:30.

The comment regarding find was incorrect: pointer comparison issue
only applies if a non-const pointer is used as the key.

Reviewed-by: Andras Lasso <lasso@queensu.ca>
Reviewed-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Isaiah Norton <inorton@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25043 3bd1e089-480b-0410-8dfb-8563597acbee