Miscellaneous improvements

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.