Blurb::
Sequence of expansion samples used in a multi-stage polynomial chaos expansion

Each level entry of the \c expansion_samples_sequence applies to one
expansion within a multi-stage expansion.  Current multi-stage
expansions that support expansion samples sequences include multilevel
and multifidelity polynomial chaos.

If adaptive refinement is active, then this sequence specifies the
starting point for each level within either an individual or
integrated refinement approach.

A corresponding scalar specification is documented at, e.g.,
\ref method-polynomial_chaos-expansion_order-expansion_samples

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See_Also::	method-polynomial_chaos-expansion_order-expansion_samples
