Move risk based point estimate out of metrics

[drewoldag]

It would make sense to port the risk based point estimate from the metrics module to a function on the pdf class itself.

[drewoldag]

This is the wrapper that will process elements of an Ensemble and pass the heavy lifting to...
https://github.com/LSSTDESC/qp/blob/main/src/qp/metrics/metrics.py#L162

... this function which does that actual work.
https://github.com/LSSTDESC/qp/blob/main/src/qp/metrics/array_metrics.py#L101

Because this is a metric that characterizes a single distribution or Ensemble, it makes sense for it to be a function that can be called like .pdf() or .cdf() on a Ensemble or distribution.