_createL_matrix(D_matrix) {
    const nodesArray = this.body.nodeIndices;
    const edgeLength = this.springLength;

    this.L_matrix = [];
    for (let i = 0; i < nodesArray.length; i++) {
      this.L_matrix[nodesArray[i]] = {};
      for (let j = 0; j < nodesArray.length; j++) {
        this.L_matrix[nodesArray[i]][nodesArray[j]] =
          edgeLength * D_matrix[nodesArray[i]][nodesArray[j]];
      }
    }
  }
Create the L matrix: edge length times shortest path
@param {object} D_matrix
@private