Get project factory cost by skill

Sums the Tessl credits of this project's finished runs (completed, failed or cancelled) by skill, in the requested window (filter[timestamp][gte] to the exclusive filter[timestamp][lt], at most 90 days, the last 7 days when neither is given) and in the window of the same length just before it, by run creation time. A run’s skill is its skillRef without a trailing @<version>, so versions of one skill add up, or (no skill) when it has none. prs counts the distinct PRs the runs link, by any rel, and creditsPerPr is the credits of the runs that link at least one PR divided by prs, null when there are none. skills is sorted by current credits, highest first. series has one point per UTC day that the current window touches, zeros included, and its points add up to current.credits.