Uniquify csv output from multigather

Hello!
Hope you and yours are doing well. I am using multigather to query protein sequences against each other, and primarily use the csv file for downstream processing. As is, if the signatures in `--query query.sig` were created from one fasta file with e.g. `--singleton`, then all results iteratively overwrite each other into the same csv file. This proposed change adds the md5sum to the query file to ensure uniqueness. Other suggestions are welcome!

(not tested yet)

Warmest,
Olga

src/sourmash/commands.py

@@ -998,9 +998,10 @@ def multigather(args):
             query_filename = query.filename
             if not query_filename:
                 # use md5sum if query.filename not properly set
-                query_filename = query.md5sum()
-
-            output_base = os.path.basename(query_filename)
+                output_base = query.md5sum()
+            else:
+                # Uniquify the output file if all signatures were made from the same file (e.g. with --singleton)
+                output_base = os.path.basename(query_filename) + "." + query.md5sum()
             output_csv = output_base + '.csv'
 
             w = None