def cell_to_cellpar(cell):
    """Returns the cell parameters [a, b, c, alpha, beta, gamma] as a
    numpy array."""
    va, vb, vc = cell
    a = np.linalg.norm(va)
    b = np.linalg.norm(vb)
    c = np.linalg.norm(vc)
    alpha = 180.0/pi*arccos(dot(vb, vc)/(b*c))
    beta  = 180.0/pi*arccos(dot(vc, va)/(c*a))
    gamma = 180.0/pi*arccos(dot(va, vb)/(a*b))
    return np.array([a, b, c, alpha, beta, gamma])
Returns the cell parameters [a, b, c, alpha, beta, gamma] as a
    numpy array.