Improve force layout with fixed positions

[flo-dup]

Please check if the PR fulfills these requirements

• 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?
When only a few positions are not fixed, force layout is

• too slow
• putting all non-fixed nodes in the middle of the diagram, leading to a messy spiderweb there

What is the new behavior (if this is a feature change)?
Force layout is now fast when only a few positions are not fixed, and the non-fixed nodes are better placed.

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

• Yes
• No

Other information:
the non-fixed position suffered

• repulsion forces from all the nodes, leading to a n^2 complexity
• attraction force to the center, which was packing all the non-fixed nodes in the middle of the diagram

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
96.8% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud