Add Connection and Callout for a Group (requiring for a new feature).

[together00]

Add Connection and Callout for a Group.

I find that we cannot add connection or callout for a group. In some cases, we need to see a group as a whole. For example, in the following figure, I want to see the group (with the nodes "operations, row/column operations, inverse") as a whole, and add a callout for the group. I think this new feature will be useful, as it is straightforward to see a group as a whole (just like a special node).