Make arguments to Lambda function for the sorting of clusters constant. Refs #1808 @30min.

This is actually the fix of a bug pointed out by Axel Puntke in Issue 1808. According to the specification of std::sort,the Compare function object must take two constant objects as arguments.

The bug went unnoticed so far probably for being corrected by most compilers automatically, but, as reported, not by gcc 4.8.2.