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.

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