Display of multiple structures

[paulsaxe]

Displaying multiple structures from e.g. and SDF file shows a couple issues:

1. The origin seems to be determined by the first structure. This can make it almost impossible to view subsequent structures if they are located elsewhere.
2. An animation button that either reversed at the end or restarted at 1 (for periodic cycles) would be useful.
3. Control over the depth of view, etc would be useful.
4. Is it possible to rotate about an axis with e.g. control key?

Probably do not want to force recentering of structures in real trajectories! It should be optional.