Expose Img3D to Python (support scripted plotting of real-space sample images)

We should provide means to visualize sample structures in real space from Python scripts.

Rationale:

• Python users will use this functionality to verify their sample model, and to generate figures for use in publications.
• As developers, we will use this functionality to generate figures for the online documentation. This will enable us to fill gaps, and ultimately to replace all Blender generated figures.

Status:

• Currently, the visualization of sample structures in real space is implemented in GUI/ba3d. It is deeply integrated with the GUI, and not accessible outside the GUI.

Work plan:

• #116 (closed), replace Vector3D by BasicVector<float>.
• Move the visualization core functionality from GUI to separate top-level component library Img3D
• #350, use polyhedral meshes from libff
• #353, use is_inside from libff
• Disentangle GUI/View/Realspace from rest of GUI
  • Particle and Layer to hold material as shared_pointer (overlap with #294 (closed))
  • Core class Material to hold QColor property
• Provide Python API.
• Generate missing artwork, and use it in GUI and in webdoc (#11).

WAITING for refactoring of material classes #616.