Litter decay

You will write the decay model of a heap of leaf litter, its half-life, and the first whole year in which the litter falls below a target mass.

The idea. The mass left is $m_0 e^{-kt}$. The half-life is $0.693 / k$, or $\ln 2 / k$.

Your tasks.

1. Finish mass_left. It gives the mass after t years.
2. Finish half_life. It gives the half-life in years.
3. Finish years_until. It gives the first whole year in which the mass is at or below the target.