Blurb::
Use the Symmetric Rank 1 update method to compute quasi-Hessians
Description::
The Symmetric Rank 1 (SR1) update (specified with the keyword \c sr1)
will be used to compute quasi-Hessians.

\f[
B_{k+1} = B_k + \frac{(y_k - B_k s_k)(y_k - B_k s_k)^T}{(y_k - B_k s_k)^T s_k}
\f]

where \f$B_k\f$ is the \f$k^{th}\f$ approximation to the Hessian, 
\f$s_k = x_{k+1} - x_k\f$ is the step and 
\f$y_k = \nabla f_{k+1} - \nabla f_k\f$ is the corresponding yield 
in the gradients. 

<b> Notes </b>

\li Initial scaling of 
\f$\frac{y_k^T y_k}{y_k^T s_k} I\f$ is used for \f$B_0\f$ prior to the first 
update. 
\li Numerical safeguarding is used
to protect against numerically small denominators within the updates. 
\li This safeguarding skips the update if 
\f$|(y_k - B_k s_k)^T s_k| < 10^{-6} ||s_k||_2 ||y_k - B_k s_k||_2\f$ 

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