Function black_scholes_pricer::bs_single::rho

pub fn rho(
    option_dir: OptionDir, 
    spot: f32, 
    strike: f32, 
    years_to_expiry: f32, 
    risk_free_rate: f32, 
    volatility: f32, 
    dividend_yield: f32
) -> f32

Single rho calculator