rstretch: $-1.1 < r < -1$ or $1 < r < +1.1$

Employ vertical grid stretching such that the distribution of vertical grid spacings is given by

$$\Delta z_k = r \Delta z_{k+1} ,$$

where $0\le \mbox{k} < \mbox{Nkmax}$. If $r<0$ then the grid is refined near the bottom, otherwise it is refined at the free-surface.