Pay attention to condition number in covariance estimation.

1. Sparse covariance estimation now uses cholmod_rcond to
detect singular Jacobians.

2. Dense covariance estimation now uses relative magnitude
of singular/eigen values to compute the pseudoinverse.

3. Truncation logic is now unified with Solver::Options::null_space_rank.

Change-Id: I095bd737510c836b4251255926190a7f31d64bce