docs: def spherocylinder_radius(V, l):
      """Return the radius of a
      [spherocylinder](http://en.wikipedia.org/wiki/Capsule_(geometry)).
  
      Parameters
      ----------
      V: float
          Volume.
      l: float
          Length of the cylinder section.
  
      Returns
      -------
      R: float
          Radius.
      """
      return np.roots([4.0 / 3.0, l, 0, -V / np.pi])[-1].real