Method precision

tests/fl/audio/detector/downbeat.hpp:50–53

    int trueNegatives = 0;   // Correctly not detecting non-downbeats

    float precision() const {
        int detected = truePositives + falsePositives;
        return detected > 0 ? static_cast<float>(truePositives) / detected : 0.0f;
    }

    float recall() const {
        int actual = truePositives + falseNegatives;