Question on the scope of groupby

[meggart]

I am trying to implement a bit more groupby logic into DiskArrayEngine and was wondering if you have any thoughts on allowing grouping on other DimArrays in the future. Currently we can only group by single dimensions, which is already useful. However, imagine I have a 3d DimArray a (lon x lat x Ti) and a 2d mask DimArray m (lon x lat) e.g. containing country codes, ideally I would like to be able to write mean.(groupby(a, m)) to get country means where both. Is something like this on your radar or does it already exist in DD and just have a different name? Just interested to hear your thoughts before continuing to work on this in DAE to make sure the interface is somewhat compatible.