2014/?????: release of version 2.0, which includes new classes for
            4-vector subtraction: NAMES GO HERE

2013/05/28: release of version 1.2.0, with a new constructor taking
            rho and rhom values, and a bug-fix in the example
2013/04/12: release of version 1.1.0, fixing an important bug in the
            choice of step size for the numerical derivatives.
2013/02/07: release of version 1.0.0
2013/02/01: release of version 0.99.1 (changes to build system)
2013/01/30: release of version 0.99