[cctbxbb] constrained hydrogen geometry

[Luc Bourhis]

I have not been able to find any facility to constraint hydrogen  
geometry, in the manner of the AFIX in ShelXL I mean. Have I missed  
them or would it need to be implemented?

In the latter case, I have tried to figure out how this could be  
done. I manage to find how position and ADP constraints are handled  
by constraint objects featuring methods to return the independent  
variables and the gradient with respect to them. Then cctbx/cctbx/ 
examples/structure_factor_derivatives_4.py shows how those features  
can be used. Would Hydrogen geometry constraint be best handled by  
mimicking those special positions constraint?

Cheers, luc