Move core packages into bundles

[Conrad Huang]

Four packages should be moved from core to bundles: ui, atomic, map and surface.

Rather than trying to move them all at once, we will add an import hook so that developers (and our own code) can use chimerax.ui, chimerax.atomic, etc. The old names (chimerax.core.ui, chimerax.core.atomic, etc.) will continue to work for now. The import hook can start giving warning messages in a few months. When all the packages have been moved, we can set a flag day (e.g., release 0.9) to disable the old names.

[Eric Pettersen]

The import-hook part of this has been implemented, so for purposes of the 0.6 release this is "done" -- moving the milestone to 0.9.

[Eric Pettersen]

All modules moved.