Deal with one-to-one functions related to quantile and interval metrics

[nikosbosse]

We currently have several one-to-one and several many-to-one functions in metrics-interval.R and metrics-quantile.R.

We need to think about whether to keep / expose the one-to-one functions to users.

Also, we could reuse some of the code, e.g. in add_coverage(). The following is there at the moment:

  data[, interval_coverage_deviation := interval_coverage - range / 100]
  data[, quantile_coverage := observed <= predicted]
  data[, quantile_coverage_deviation := quantile_coverage - quantile]

[nikosbosse]

Current thinking here is to not expose functions to users for now and rethink later for future releases