Function pearson 

pub fn pearson(x: &[f64], y: &[f64]) -> Result<f64, StatsError>

Pearson correlation of two equal-length samples.

Errors

• StatsError::LengthMismatch if the inputs differ in length.
• StatsError::EmptyInput if empty.
• StatsError::ZeroVariance if either sample is constant (correlation undefined rather than a silent NaN).