Blurb:: Employ a hierarchical sparse grid construction

Description::

Stochastic collocation with sparse grids can employ either a nodal or hierarchical
formulation.  The hierarchical formulation requires nested interpolation rules
as it utilizes increments in collocation points along with hierarchical weights
defined from a hierarchical basis formulation.

This keyword requests the hierarchical formulation, which has benefits
for adaptive refinement by supporting direct computation of increments
in targeted statistics (e.g. mean, covariance, reliability indices),
avoiding loss of precision from subtractive cancellation.

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