code: public RandomVariable  getZeroRate(double maturity)
  	{
  		if(maturity == 0) {
  			return this.getZeroRate(1.0E-14);
  		}
  
  		return getDiscountFactor(maturity).log().div(-maturity);
  	}
docs: Returns the zero rate for a given maturity, i.e., -ln(df(T)) / T where T is the given maturity and df(T) is
  the discount factor at time $T$.
  
  @param maturity The given maturity.
  @return The zero rate.