Blurb:: Normalize by sample mean of gradient norms

Description::
For each reponse function \f$f_i\f$, normalize its gradient data by
the mean \f$m_i\f$ of the gradient two-norms taken across samples \f$
j=1,...,J \f$:

\f[ m_i = \frac{1}{J} \sum_{j=1}^{J}{\| \nabla^j f_i \|_2} \f]

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