_updateE_matrix(m) {
    const nodesArray = this.body.nodeIndices;
    const nodes = this.body.nodes;
    const colm = this.E_matrix[m];
    const kcolm = this.K_matrix[m];
    const lcolm = this.L_matrix[m];
    const x_m = nodes[m].x;
    const y_m = nodes[m].y;
    let dE_dx = 0;
    let dE_dy = 0;
    for (let iIdx = 0; iIdx < nodesArray.length; iIdx++) {
      const i = nodesArray[iIdx];
      if (i !== m) {
        //Keep old energy value for sum modification below
        const cell = colm[iIdx];
        const oldDx = cell[0];
        const oldDy = cell[1];

        //Calc new energy:
        const x_i = nodes[i].x;
        const y_i = nodes[i].y;
        const denominator =
          1.0 / Math.sqrt((x_m - x_i) ** 2 + (y_m - y_i) ** 2);
        const dx =
          kcolm[i] * (x_m - x_i - lcolm[i] * (x_m - x_i) * denominator);
        const dy =
          kcolm[i] * (y_m - y_i - lcolm[i] * (y_m - y_i) * denominator);
        colm[iIdx] = [dx, dy];
        dE_dx += dx;
        dE_dy += dy;

        //add new energy to sum of each column
        const sum = this.E_sums[i];
        sum[0] += dx - oldDx;
        sum[1] += dy - oldDy;
      }
    }
    //Store sum at -1 index
    this.E_sums[m] = [dE_dx, dE_dy];
  }
Update method, just doing single column (rows are auto-updated) (update all sums)
@param {number} m
@private