Blurb:: Period at which to update derivative-based proposal covariance

Description::
For derivative-based proposal covariance, this specifies the period
(number of accepted MCMC samples) after which the proposal covariance
is updated using derivative values at the current chain point.

<b> Default Behavior </b>

When \c update_period is not specified, derivatives will inform the
proposal covariance at the start of the chain, but not updated
further.

<b> Usage Tips </b>

The \c update_period should be tailored to the size of the total chain,
accounting for the relative expense of derivative-based proposal updates.

Topics::

Examples::
\verbatim
method,
        bayes_calibration queso
          samples = 2000 seed = 348
          delayed_rejection
          emulator pce sparse_grid_level = 2
          proposal_covariance derivatives
            update_period = 40  # update proposal covariance every 40 points
\endverbatim

Theory::
Faq::
See_Also::
