Specifies a filename prefix for saving clusters. Supported
by cluster_fast only. The sequences in each
cluster are written to a separate FASTA file. The first sequence in the file is
always the centroid. The filename is constructed by appending an integer cluster
number 0, 1, 2... to the string specified by -clusters.
The value of -clusters may include a path, which must
exist. In the example below, the cluster filenames will be cluster_dir/c_0,
usearch -cluster_fast seqs.fa -id 0.90 -clusters