manysketch does not set name properly for multiple sketches per entry

[bluegenes]

naming bug likely introduced in #184

set_name likely not working as intended. I think it sets the name for the first signature (e.g. k=21) but not for any subsequent (e.g. k=31,51).

 } else if !set_name {
      sig.set_name(name);
   // sourmash sets filename to last filename if merging fastas
      sig.set_filename(last_filename.as_str());
     set_name = true;