Benchmark methodology toolkit (BD-rate + provenance)

A dependency-free (pure standard-library) Python tool implementing the two things this methodology article describes, so a reader can both reproduce our BD-rate math and produce a citable provenance record of their own. (1) BD-RATE DONE RIGHT: computes Bjontegaard delta-rate from two sets of (bitrate, quality) points using a SHAPE-PRESERVING piecewise-cubic interpolation (PCHIP-style monotone Hermite, which cannot overshoot its data points the way the original single cubic does), integration in