Function compare 

pub fn compare(
    tool: &Tool,
    metric: FlightMetric,
    reference: ReferenceReading,
    measured: Option<f64>,
) -> MetricOutcome

Compares hpr’s measured value of metric with a reference reading from tool.

• A metric whose meaning in tool’s version is not measured is withheld (Loft lesson L80), and so is every metric of an aborted reference flight.
• A reference None or NaN means its flight has no value; hpr’s None means the same, but a NaN or infinity from hpr, or an infinity from the reference, is a failure.
• For a metric taken at an event, no value on both sides withholds it; on one side only, it fails, since the flights disagree about what happened (Loft lesson L81: never 0).
• For a metric every complete flight has, such as the apogee, a missing value fails.

measured must be taken by the Definition that definition gives for tool: the last deployment, say, not the first.