Add swarm to pick_otus.py

We already have `uclust_otu_id_prefix` and `sumaclust_otu_id_prefix`... now we are adding `swarm_otu_id_prefix`. Probably worth to revisit this and rename it to `denovo_otu_id_prefix` and use the same parameter for the three tools? `pick_otus.py` already has a lot of options, so reducing them will be a good idea....