Delay cctbx import until inevitable.
Use base class for object type check.

The original check would always fail, because cctbx.crystal is a module.
special_position_settings is a base class for
cctbx.xray.structure.structure and cctbx.neutron.structure.
Not sure if this adapter works, but at least has a better chance now.
Clean trailing blank lines.

No change in the code.