Plane objects were designed primarily to handle crystallographic planes but
they should provide useful functionality even in the absence of 3D periodic
objects (cells). Plane objects can own directions, atoms, orbitals and texts.
The low-level engine infra-structure required to implement plane objects is
fully implemented but everything else is not done yet. Plane objects must handle
Miller and Miller-Bravais indices and all the related crystallographic relations
provided by direct- and reciprocal-lattice representations. Users should be able
to choose planes of atoms in cells, clusters or molecules, automatically producing
plane objects reproducing those planes, including the atoms and its child, bonds,
orbitals and texts, and then handle these new plane objects as totally independent
viewing and building tools.