Allow accessing a TaskGroup's members via []

[Dev-iL]

Minor feature (syntactic sugar): we can now do e.g. some_group["some_task"] to access the children of a task group regardless of the prefix_group_id setting.

Motivation: Discussion in AIR201's PR that made me aware that the shortest way to access child references is not that short - so I made it shorter and more natural (at least to me).

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[potiuk]

LGTM, but likely @uranusjr would be good to take a look

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[eladkal]

LGTM
cc @uranusjr as you are most familiar with this area