Decreasing threshold at which L-BFGS Hessian is updated.

- Decreasing threshold at which L-BFGS Hessian is updated from 1e-10
  to 1e-14 results in a very significant improvement in NIST scores
  (43 -> 53 for CUBIC).
- Adding comment in FindPolynomialRoots() explaining why behaviour
  is correct.

Change-Id: If668e087e7a86d29659aa74e8528b192b604c841