CellNEST pairwise distance computation out of memory

Hi,

Thanks for the great package. CellNEST runs out of memory when calculating the pairwise distance matrix on single-cell resolution ST data. This is mentioned in #6. I think this is due to storing the whole cellXcell distance matrix. If we are using knn for single-cell resolution, can we not just store a cellXk matrix instead? Looking forward to your inputs.

Thanks,
Harry