reuse previously computed dephier in subsequent calls to fill-spil-merge

[jkingslake]

Hi, Thanks for developing a fantastic tool! I am making some progress on applying it to flow across Antarctic ice shelves (e.g., here) and was wondering about a feature that could help speed things up.

My workflow (described here) involves running fill-spill-merge over the same DEM many times. In each iteration I use a different input of water (different magnitude and different locations within the DEM). This means I am computing the depression hierarchy (on this line) every time.

Would it be possible to instead compute it once, then save it to disk, perhaps with this code, and reload it each time instead of recomputing it?

Thanks for you help!