[sld-viewer] Simplify computations for navigation arrow

[flo-dup]

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? Refactor / simplification

What is the current behavior?

• the arrow ordinate is computed by filling two id-ordinate maps in a loop over all navigable nodes (nodes with a non-null nextVId): one map for computing the max ordinate for each node id, one map for computing the min ordinate for each node id
• the unique list of vls displayed is computed by looping over all the nodes
• the computation of the list of unique vls is done with a complex indexof (yet classic!) which is not commented

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

• y is directly read like x already was. Indeed, the same element id shouldn't be twice in the metadata, and even if it appears twice by mistake the querySelector would return the same svg element, hence the same ordinate.
• the unique list of vls displayed is computed by looping over the navigable nodes: if one navigable node points to a vl displayed there is a navigable node in this vl (on the other side)
• the computation of the list of unique vls is done with a Set

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

• [ ] Yes
• [x] No

[flo-dup]

Indeed! I missed the fact that the map was indexed by the voltage level id...

The SVG constructed by powsybl-single-line-diagram has always y position equals for all the feeder nodes within the same voltage level: it corresponds to the top extern cell height or the bottom extern cell heigh which are both constant within a voltage level. So we could keep this simplification anyway

[CBiasuzzi]

It's not always the case, apparently. In the SVG created with this code, the two bottom elements have different y:

Network network = Importer.find("CGMES")
        .importData(CgmesConformity1Catalog.microGridBaseCaseBE().dataSource(), new NetworkFactoryImpl(), null);
//let's create an SLD for the 4th VL
SingleLineDiagram.draw(network, "b10b171b-3bc5-4849-bb1f-61ed9ea1ec7c", "/tmp/sld-example.svg");

with the new logic for the diagram-viewer, the bottom arrows are not aligned: