Seed viability and categories

You will write functions for the viability of a stored seed lot, for the time until the lot must be grown out, and for the threatened categories of the IUCN Red List.

The idea. In a model, viability is start * 0.5 ** (years / half_life). The time to a limit is half_life * log(limit / start) / log(0.5). The threatened categories are CR, EN and VU.

Your tasks.

1. Finish viability. It gives the per cent of viable seeds after some years.
2. Finish grow_out. It gives the years until the viability falls to the limit.
3. Finish is_threatened. It gives True for "CR", "EN" and "VU", and False for any other code.