symmetrization to avoid symmetry breaking

[psi-rking]

Symmetrize geometry step in Cartesian coordinates to maintain exact, point group symmetry.

[psi-rking]

The SF4 test case in internal coordinates goes awry, I think, due to a step not being quite C2v in step 7-8. So if this problem is fixed, that test may be successful.

See second test in tests/test_sf4_quasilinear.py