Uses the Black-Scholes model to calculate the "delta" of a European put option struck at @strike on an asset with price @price.
(The delta of an option is the rate of change of its price with respect to the price of the underlying asset.)
@volatility is the annualized volatility, in percent, of the asset for the period through to the exercise date. @days_to_maturity the number of days to exercise, and @rate is the risk-free interest rate to the exercise date, in percent.
The returned value will be expressed as the rate of change of option value per unit change in @price.
opt_bs_call, opt_bs_put, opt_bs_call_deltaopt_bs_call_rho, opt_bs_put_rho, opt_bs_call_theta, opt_bs_put_theta, opt_bs_vega, opt_bs_gamma.