public String toString() {
        String s = "";
        s += String.format("%.2f N + %.2f", slope(), intercept());
        return s + "  (R^2 = " + String.format("%.3f", R2()) + ")";
    }
Returns a string representation of the simple linear regression model.
@return a string representation of the simple linear regression model,
  including the best-fit line and the coefficient of determination <em>R</em><sup>2</sup>
library/src/main/java/com/lorentzos/flingswipe/LinearRegression.java