multigather only saves first match to CSV

[ctb]

when running:

sourmash multigather --query SRR606249-abund-10k.sig.zip --db out.txt -k 31 --threshold-bp=0

the resulting file SRR606249.trim.fq.gz.csv (which is based on the query signature filename property... ugh) only contains the first result.

this code is at fault in commands.py::multigather:

            w = None
            with FileOutputCSV(output_csv) as fp:
                for result in found:
                    if w is None:
                        w = result.init_dictwriter(fp)
                        result.write(w)

where the last line should be deindented so that it is run every time through the loop.