Check if a circular link is the only circular link for both its source and target node
function onlyCircularLink (link) {
    var nodeSourceLinks = link.source.sourceLinks
    var sourceCount = 0
    nodeSourceLinks.forEach(function (l) {
      sourceCount = l.circular ? sourceCount + 1 : sourceCount
    })

    var nodeTargetLinks = link.target.targetLinks
    var targetCount = 0
    nodeTargetLinks.forEach(function (l) {
      targetCount = l.circular ? targetCount + 1 : targetCount
    })

    if (sourceCount > 1 || targetCount > 1) {
      return false
    } else {
      return true
    }
  }