code: function _tagOrphanNodes(nodes, linksMatrix) {
      return Object.keys(nodes).reduce((acc, nodeId) => {
          const { inDegree, outDegree } = computeNodeDegree(nodeId, linksMatrix);
          const node = nodes[nodeId];
          const taggedNode = inDegree === 0 && outDegree === 0 ? { ...node, _orphan: true } : node;
  
          acc[nodeId] = taggedNode;
  
          return acc;
      }, {});
  }
docs: Tags orphan nodes with a `_orphan` flag.
  @param {Object.<string, Object>} nodes - nodes mapped by their id.
  @param {Object.<string, Object>} linksMatrix - an object containing a matrix of connections of the graph, for each nodeId,
  there is an object that maps adjacent nodes ids (string) and their values (number).
  @returns {Object.<string, Object>} same input nodes structure with tagged orphans nodes where applicable.
  @memberof Graph/helper