Feature description

RDataFrame currently supports n-dimensional histograms using HistoND() method. However, this method will create THnD histogram which consumes too much memory.

I would like to add HistoNSparseD() method which does the same with THnSparseD histogram. This will also require to add THnSparseD model into

https://root.cern/doc/v628/HistoModels_8hxx_source.html

I believe that implementation will be identical to THnD histogram. Only THnD will be replaced by THnSparseD.

Alternatives considered

No response

Additional context

I would like to use it for physics analysis where I will define 6 or 8-dimensional histograms. I would like to produce them with finer binning for binning optimization.