_getEnergy(m) {
    const [dE_dx, dE_dy] = this.E_sums[m];
    const delta_m = Math.sqrt(dE_dx ** 2 + dE_dy ** 2);
    return [delta_m, dE_dx, dE_dy];
  }
calculate the energy of a single node
@param {Node.id} m
@returns {number[]}
@private