Draft: Move B2ll(ll) line isolation outputs to function from !2973

⚠️ DO NOT MERGE UNTIL BOTH !2948 (merged) AND !2973 (merged) ARE MERGED ⚠️

This MR enhances the isolation output implementation in !2948 (merged), using the function parent_and_children_isolation implemented in !2973 (merged).

Functionally this should make no difference to the outputs; however, this will tidy the B2ll and, particularly, the B2llll lines. It is therefore also not necessary for this MR to be considered in the context of 2024data-selections.

To avoid delays to either !2948 (merged) or !2973 (merged), this is entirely on hold until both are merged.