Fit a line by least squares

You will write the least-squares slope and intercept from the sums about the means, and a prediction function. The checker uses the canteen data and compares with the statistics module.

The idea. $b = \dfrac{S_{xy}}{S_{xx}}$, where $S_{xy} = \sum (x - \bar{x})(y - \bar{y})$ and $S_{xx} = \sum (x - \bar{x})^2$. Then $a = \bar{y} - b\bar{x}$, and a prediction is $a + bx$.

Your tasks.

1. Finish slope(xs, ys).
2. Finish intercept(xs, ys). Use slope.
3. Finish predict(xs, ys, x).