Option to return sparse arrays from `sc.get.aggregate`

What kind of feature would you like to request?

Additional function parameters / changed functionality / changed defaults?

Please describe your wishes

@Intron7, found a use case 😆

It could be nice for sc.get.aggregate to be able to return sparse matrices where we don't expect the aggregation to return very dense data.

Previously discussed in:

• https://github.com/scverse/scanpy/issues/2892

Usecases include:

• Taking the max for multiple reports of a genes (sc.get.aggregate(adata, "probe_target", "max"), e.g. https://discourse.scverse.org/t/merging-identical-genes-from-10x-fixed-scrna/2142)
  • (note: max is not currently implemented)
• Small aggregations, e.g. only summing neighbors

This would require both api design choices for what the argument is called, and efficient implementations for both dense and sparse results (python-graphblas could be useful here)