Fix extern cells on two (or more) subsections

Please check if the PR fulfills these requirements

[x] The commit message follows our guidelines
[x] Tests for the changes have been added (for bug fixes / features)
[ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem? No

What kind of change does this PR introduce? Bug fix

What is the current behavior? Extern cell on two subsections is detected and its rootBlock replaced by UndefinedBlock

What is the new behavior (if this is a feature change)? Extern cell on two subsections is detected and is cut into pieces in order to have

one extern cell on one single subsection
several arch cells based on the legs linked to other subsections, each arch cell referring to the legBlock kept for coordinate calculation

Does this PR introduce a breaking change or deprecate an API?

[ ] Yes
[x] No

Other information: Due to the changes involved, some side-effects appeared which needed the following change/fix: The spanning of crossover intern cells now corresponds to the effective width and not only to a left part containing all equipments; this leads to equipments to be centred on the horizontal (body) part.

powsybl / powsybl-diagram

Fix extern cells on two (or more) subsections #597

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