Three measures of spread

You will write the mean absolute deviation and the two standard deviations yourself, and the checker compares them with the statistics module.

The idea. The deviations are the values minus the mean. The MAD averages their sizes. The population SD divides the sum of the squared deviations by $n$, and the sample SD divides it by $n - 1$, before the square root.

Your tasks.

1. Finish mad(data).
2. Finish pop_sd(data).
3. Finish sample_sd(data).