Resolve "EXTERN: Slicing a Container returns a List, not a Container"

Summary

Slicing a Container used to return a list instead of a Container (issue #183 (closed), issue #104 on GitLab.com). This is fixed by adding a Container.__getitem__ method.

Focus

Checkout Container.__getitem__ and the corresponding test test_container_slicing in test_container.py.

Test Environment

Regular unittests.

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