TODO

CURRENT
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Make ribbon normals consistent eg all outward facing. Will need to
calc the ribbon centroid for this & flip normals if inward facing.

Instead of doing a costly get_elem_data(), each atom has a pointer to
the relevant (global/local) static elem_pak to use for this atom.

Long range:
Group all non atom/bond stuff in generic object listing.

GDIS now automatically distiguishes rhombo & hex type cells, but
only if spacename is specified (searches for 'R'). Need to be 
able to do the same for space number (ie find the range)

SOF entry for shells. (what about when one shell is near multiple cores?)

The use of bash (exec -> "sh") is the cause of the pid
difference when spawning gulp jobs via the old method under linux.
The mac uses zsh (or something else) which doesn't do this.

POVRay has problems if two like objects are placed at the same position,
eg partially occ. sites will cause render to fail. Perhaps some sort of
redundancy checker should be run before the .pov file is generated.

Do nicer looking ellipses instead of circles for angle measurements.

POVray sometimes puts ugly white dots on a rendered sphere. Only
noticed it happening occasionaly on hydrogen (ie on the grey shaded
regions of white spheres). OpenGL sometimes does this too - but AFAIK 
only on the cylinders in B&S or cylinder drawing mode.

Add scrolled win for viewing atoms & coords & status flags of a model.
Could then undelete/edit atoms directly. This could be a general 'data'
child of a model that contains periodic or not + sginfo/pbc etc that
can also be edited & auto updated by gdis.

Make an option to enable the user to either: save transformed model
(ie with all the centering & rotations done in gdis), or just save
the original configuration. The difference is mainly between using
(x,y,z) and (rx,ry,rz) as save coordinates - but also the centroid.

THINK ABOUT - an undo function (store past operations that have inverses,
              can undo an operation if it has an inverse).
            - internal scripting (guile?), so that gdis can be run to do
              operations on files without a GUI coming up.