Alembic export: added support for writing dupli-groups

This supports our common character animation workflow, where a character,
its rig, and the custom bone shapes are all part of a group. This group
is then linked into the scene, the rig is proxified and animated. Such
a group can now be exported. Use "Renderable objects only" to prevent
writing the custom bone shapes to the Alembic file.

Alembic export: unified code of exploreTransform and exploreObject.

Alembic export: removed superfluous export_object() check.

The exploreTransform() function already does this check, and the check
isn't necessary when the object type isn't exported anyway.