Ultimately, the objective is for the user to be able to get the raw
sampled metric values, or the mean and standard deviation of the sampled
metric values, or a series of quantiles of the sampled metric values.
There are still problems with the standard deviation on rtensor, and
the computation of the quantiles does not work on n-dim expressions yet.
So the second and third options are not possible yet, so only the raw
values can be returned. Nonetheless, the machinery and the choice of
where to introduce the summary functionality could be implemented,
which is the purpose of this commit. A new parameter of the bootstrap
experiment called "summary" is added: it can be given a value of 0 (to
get the raw values). In the future, it would also take a value of 1 for
mean+std, and 2 for quantiles.