Blurb::
Required or expected accuracy in optimization variables.

Description::
Unlike it's use as a stopping criteria in other methods, \c
variable_tolerance is used to communicate the accuracy of the
optimization variables to COBYLA.  It represents the mimimum distance
from the simplex within which linear approximation subproblems can be
trusted to be sufficiently representative of the true problem.  It is
analagous to the minimum trust-region size used in trust-region
methods.  Note that, per COBYLA documentation, the level of accuracy
is not guaranteed.

<b> Default Behavior </b>

The default value is 0.0001.

<b> Additional Discussion </b>

While \c variable_tolerance is not a stopping criteria for COBYLA, we
note that it may have a side effect regarding convergence.  In
particular, if lower accuracy is required of the optimization
variables, it may stop sooner than if higher accuracy is required.

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