Miscellaneous improvements

[jhlegarreta]

Miscellaneous improvement list transferred from the README file at commit cc59432:

• Implement multi-threading.
• Add a real world example.
• Investigate Jensen unexpected behaviour in simple example.
• Currently, the point set metrics are not symmetric since the iterations is only over one of the point sets. Consider adding symmetric versions. Or document the asymmetric behaviour well.
• A decorator to the PointSetToPointSetMetricv4 that overrides the accumulation of the value and derivative computation to use a trimmed number of points is in the module as well, but is not not functional yet. The issue is access to protected members in the base class that I think would need to be fixed in the base class.