Blurb:: Specification of an unordered ensemble of low-fidelity approximations

Description::
A \c non_hierarchical surrogate model manages an unordered set of
low-fidelity model approximations, each of which may include hyper-parameter
resolution controls (in the case of a simulation model) or additional model
recursions.

Any corresponding sequence specifications within methods (e.g., \c
quadrature_order_sequence, \c sparse_grid_level_sequence, \c
expansion_order_sequence, etc. within stochastic expansion methods)
should be synchronized with the order in the model listing.

Internal to the non-hierarchical model, subsets of the model ensemble
may be active for any given evaluation, as dictated by the iterative
algorithm in use.

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Topics::
Examples::
\verbatim
model,
	id_model = 'NONHIERARCH'
	surrogate non_hierarchical
	  unordered_model_fidelities = 'LF1' 'LF2'
	  truth_model_pointer = 'HF'

model,
	id_model = 'LF1'
	simulation
	  interface_pointer = 'LF1_DRIVER'
          solution_level_cost = 1.

model,
	id_model = 'LF2'
	simulation
	  interface_pointer = 'LF2_DRIVER'
          solution_level_cost = 2.4

model,
	id_model = 'HF'
	simulation
	  interface_pointer = 'HF_DRIVER'
          solution_level_cost = 256.
\endverbatim

Theory::
Faq::
See_Also:: model-surrogate-hierarchical
