Fixing limit of filling sparse matrix generator.

for size in sizes:

   nnz_per_row = 1

   while nnz_per_row < 50 and nnz_per_row <= size:

   while nnz_per_row < 256 and nnz_per_row <= size:

      print( f"Creating matrix with {nnz_per_row} non-zeros per row")

      rows = []

      columns = []