[cctbxbb] Tidy up CCTBX

[Marcin Wojdyr]

Hi Rob,

> What is currently complicating this is the inclusion of the dxtbx module
> within the cctbx_project.

and in particular that iotbx depends on it, as it was discussed half a
year ago in a thread: "shall iotbx really depends on dxtbx?"

> If all a user
> wants is to calculate allowed origins and symmetry sites for a given space
> group then an alternative to CCTBX might be preferable rather than having to
> massively expand their project [...]

Yes! Please don't change it :-)
It's one of the selling points for a library I'm working on (
https://project-gemmi.github.io/ ).

Cheers,
Marcin