[cctbxbb] differential evolution

[Nikolay Mladenov]

The implementation of the algorithm seems wrong.
As I understand it, the new generation has to be generated based on
the old one,
at which point every member of the old generation is replaced or not
by its child.
In the cctbx implementation every member is replaces as soon as it
gives birth to a better child.
I am not sure what the implications are but it probably reduces the gene pool.

Nikolay.