Having a cache to store downloaded data would be useful among other things for building the gallery where different examples may want to download the same DEM.

In pytopotoolbox, we save data downloaded from the equivalents of readexample and readopentopo to a platform-specific cache directory and then check whether we have already downloaded the data before requesting it again. The cache is not automatically cleared; instead, we provide users with a clear_cache function to delete things from their cache.