Zones of a reserve

You will write functions for the share of each zone of a model biosphere reserve, for the area of the hotspots, and for the extinctions that protection avoids.

The idea. A reserve is a set of nested squares. The core is a square of side core. The core and buffer make a square of side core + 2 * buffer. The whole reserve adds the transition on both sides.

Your tasks.

1. Finish zone_shares. It gives the share of the core, the buffer and the transition, in per cent, in a list.
2. Finish hotspot_area. It gives the area in million square kilometres from the land and the share in per cent.
3. Finish avoided. It gives the extinctions avoided from the expected loss and the reduction in per cent.