Resampling stages for the summarizers

[ztq1996]

In cosmological analysis, a common task is to sample realizations of n(z) from the sample redshift distribution (e.g., SOM/multivariate distribution). This function is now realized in the summarizer classes, however, it makes sense to have a "resampling" that allows the users to sample from the distribution regardless of which summarizer the user is using.

There are several benefits:

1. avoid duplicating some of the resampling/bootstrapping functionality in summarizers.
2. Unify the API for resampling