The population ledger

You will write functions for the next size of a population, for its sex ratio and for the kind of its age pyramid.

The idea. The next size is $N + (B + I) - (D + E)$. The sex ratio is the females for every $1000$ males. A pyramid is expanding if the pre-reproductive share is more than $5$ points above the reproductive share, stable if they are within $5$ points, and declining otherwise.

Your tasks.

1. Finish next_population. It gives the size at the end of the year.
2. Finish sex_ratio. It gives the females for every $1000$ males.
3. Finish kind. It gives "expanding", "stable" or "declining".