v2.1.0

• Add forceCenter.strength. #81
• Add forceSimulation.randomSource.
• All built-in forces are now fully deterministic (including “jiggling” coincident nodes). #121
• Improve the default phyllotaxis layout slightly by offsetting by one half-radius.
• Improve the error message when a link references an unknown node.
• force.initialize is now passed a random source.
• Use exact paths in imports.
• Add more tests. #17