filter_nb_supporting_cells should be refactored.

[dputhier]

Some part of the code is obfuscating. E.g.

  # Compute the percentage of genes expressed per cell
  cur_clust[cur_clust > 0 | cur_clust < 0] <- 1

The rest of the code could be simply replace by a call to the indexing function.

[dputhier]

Done.