Fix one leg intern cells

Please check if the PR fulfills these requirements

[x] The commit message follows our guidelines
[ ] 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? One-leg intern cells, which are intern cells which are displayed as a single vertical line, forces the corresponding busbars to be parallel, which used to lead to unexpected behaviour (mixing components on one of the two bars). Since PR #573, an exception is thrown.

Example of one-leg intern cell from unit tests: one-leg

What is the new behavior (if this is a feature change)?

Wannabe one-leg intern cells are detected when organizing blocks like previously but the corresponding shape is now MAYBE_ONE_LEG
Then, when building the LegBusSets, the maybe-one-leg cell is
- either integrated in an existing LegBusSet if all its busNodes are contained in that one, its shape is then confirmed ONE_LEG
- or split into two LegBlocks based on the LegBusSets it is attached to and its shape is either marked UNDEFINED or MAYBE_FLAT
- or split into two LegBlocks based on the sectionIndex of their BusNode and its shape is either marked UNDEFINED or MAYBE_FLAT
- or marked as ONE_LEG
Later, when creating subsections, if we detect that the maybe-one-leg cell which was split into two parts is fully included in a subsection, we finally set its shape to ONE_LEG

Note that when splitting in two parts the subBlocks of the internCell, we favour the splitting based on LegBusSets as they can deal with complex subsections which contain more than one sectionIndex (e.g., two parallel busbars with different lengths).

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

[ ] Yes
[x] No

powsybl / powsybl-diagram

Fix one leg intern cells #595

Quality Gate passed