Get project factory cost

Sums the Tessl credits of this project's finished runs on factory PRs 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 factory PR is one a project run produced; its earliest producing run is the opening run, which always counts. Every other run on it created before the PR merged is a follow-up; one created at or after the merge is not counted, and a PR with no recorded merge counts every follow-up. prsOpened counts the PRs whose opening run is in the window. A run on several PRs splits its credits evenly between them. series has one point per UTC day that the current window touches, zeros included, and its points, each split into firstRunCredits and followUpCredits, add up to current. GET .../factory/metrics/cost/prs lists the PRs behind these totals and .../cost/breakdown groups them.