[BUGFIX] Update `OnboardingDataAssistant` to use `oneshot` estimator

[NathanFarmer]

Changes proposed in this pull request:

• Update OnboardingDataAssistant to use oneshot estimator with linear interpolation method by default

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[alexsherstinsky]

@NathanFarmer This does not provide the Estimator defaults -- let us discuss and consider all changes that need to be made (including the name change from oneshot to something more representative of our use of quantiles). Maybe something from the BoxPlot terminology could be brought to bear (https://en.wikipedia.org/wiki/John_Tukey). :-). Thanks!