Code for DEM statistics

[fmaussion]

@matthiasdusch do you have the code somewhere that produced the summary statistics on https://rgitools.readthedocs.io/en/latest/dems_subregions.html ?

@afisc is taking over for RGI7 and this would help us to get started

[matthiasdusch]

took me a bit of digging but I found it :-)