Calculate VCV from the results of minimize function

[FuZhiyu]

In the notebook MLest, we use
vcv_mle = results.hess_inv * OffDiagNeg
to obtain the VCV. However, results.hess_inv is not a ndarray but a scipy.optimize.LbfgsInvHessProduct object, which will interpret '*' as dot multiplication instead of pointwise multiplication, thus generating irregular VCV result.
Instead, we should use:
vcv_mle = results.hess_inv.todense() * OffDiagNeg
i.e., transform it to a dense matrix, and then do the pointwise multiplication.

[rickecon]

@FuZhiyu . That is correct. The .todense() method will extract a sparse matrix from the results output. Then do the pointwise transformation.