Smaller node for single-bus voltage level

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? Feature

What is the current behavior? (You can also link to an open issue here) Voltage levels are all of the same size (except fictitious ones)

What is the new behavior (if this is a feature change)? Voltage levels are bigger if more than one bus

screenshot

Does this PR introduce a breaking change or deprecate an API? No

Other information: One could think of a radius proportional to the number of buses, nonetheless this requires more code changes (expecially the forking edges)

powsybl / powsybl-network-area-diagram

Smaller node for single-bus voltage level #55