Unit cells are not fully supported

[avmarchenko]

Currently only a simple cubic unit cell is displayed (if available). This is a complete hack and should be fixed. Relevant code is in exatomic/widgets/traits.py:frame_traits and js/src/appthree.js:add_unit_axis. There is a comment that says "Hack to also add unit..." (in that function) which is the place to start on the frontend side of things.

This code (and the Python code in traits.py) should be generalized to support an arbitrary cell and to support unique cells for every frame in a multiframe universe.