[cctbxbb] Possible contributions to the cctbx

This is a spin-off from the thread about constrained hydrogen refinement. On the todo list of our EPSRC project, we have a few more items which I think would fit well in the cctbx. 1) anharmonic thermal displacements. Not only the Gram-Charlier ones but also about the modelling of atoms disordered over a circle or a line (as done in Crystals [1]). 2) multipolar scattering factors, for charge density studies. [1] L. Schröder, D.J. Watkin, A. Cousson, R.I. Cooper, and W. Paulus. Crystals enhancements: refinement of atoms continuously disordered along a line, on a ring or on the surface of a sphere. J. Appl. Cryst., 37:545–550, 2004. Best wishes, Luc