add multiple sequence alignment Pipeline, with parallel support

[gregcaporaso]

This will be an example of a more complex combine Method, because alignments of alignments will need to be performed as results of the apply accumulate. Some smart splitting may also be relevant, for example to group similar sequences into each split - this should result in higher quality alignments resulting from each apply step, and then ideally the resulting alignments will align to each other better because they represent profiles of closely related sequences. This would more or less be the guide tree approach.