[BUG] Bug fix to use multiple-seq-alignment command

[suzuki-shm]

When I used deblur multiple-seq-alignment command, I face an error as follow:

$ deblur multiple-seq-alignment test.fasta.no_artifacts test.fasta.no_artifacts.ma
Traceback (most recent call last):
  File "INSTALL_DIR/bin/deblur", line 684, in <module>
    deblur_cmds()
  File "LIBRARY_DIR/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "LIBRARY_DIR/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "LIBRARY_DIR/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "LIBRARY_DIR/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "LIBRARY_DIR/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "INSTALL_DIR/bin/deblur", line 328, in multiple_seq_alignment
    f.write(alignment.to_fasta())
AttributeError: 'str' object has no attribute 'to_fasta'

This error happens because multiple_seq_alignment returns file path of the output directory and it is inserted into alignment. As t his variable is just a string type variable, it cannot use to_fasta method.

I fixed this problem removing the procedure to save the results into the specified path as mafft result has already been saved in seqs_fp + "msa" before finishing the function by system-called mafft. If it is required to use a specified output of multiple-seq-alignment command, please modify this additionally.

[coveralls]

Coverage remained the same at 88.911% when pulling 4bcd7cb on TaskeHAMANO:feature/msa_cmd into 4b4badc on biocore:master.