How points will be selected: one of `UCB`, `TS`,
`MAP`, `Oldest`, `Random`, or `Lowest`.
`UCB` uses Upper Confidence Bound estimates for the parameters.
`TS` uses Thompson sampling, a random sample from the posterior.
`MAP` uses maximum a posteriori parameter estimates.
`Oldest` adds the block that has been available the longest.
`Random` adds a random block.
`Lowest` adds the block with the lowest sum of index levels.
`UCB` and `TS` are based on bandit algorithms and account for uncertainty
in the parameter estimates, but are the slowest.
`MAP` is fast but doesn't account for parameter uncertainty.
The other three are naive methods that are not adaptive and won't
perform well.