Gecode 6.0.0

This major release fixes many bugs, adds quite some new functionality, and changes how cloning works (most likely the biggest change we ever did to Gecode): the Boolean share argument is not any longer needed (see how to change your programs in "How to Change to Gecode 6.0.0"). The added functionality includes experimental support for the CPProfiler, classes for lexicographic optimization (where the cost is defined by an array of variables), a new much faster implementation of extensional constraints using tuple sets, and support for tracing search engines.