NaN value for PrimitiveCell.for_rhombohedral_lattice

[kitchoi]

when the angle alpha gets close to 120 degree, the z coordinate of the third primitive vector gets vanishingly small. Eventually np.sqrt(sina**2 - ((cosa-cosa**2) / sina)**2) here is trying to compute the square-root of a negative value.

Edited:
Similar problem with for_triclinic_lattice
Should raise a ValueError when the quantity within sqrt is less than 0