Blurb::
Sample allocation approach for multilevel expansions
Description::
Multilevel expansions, including regression-based polynomial chaos expansion
(PCE) and function train (FT) expansions, require a sample allocation strategy.
Three options are currently available:


- allocation based on assuming a convergence rate for the estimator variance (for regression PCE)
- restricted isometry property (RIP) sampling (for regression PCE via compressed sensing)
- rank sampling (for FT)

*Default Behavior*

Current defaults, when ``allocation_control`` is not specified, are
estimator variance for PCE and rank sampling for FT.
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