generalization of aligner functions to support aligning alignments

The global aligners can now align pairs of sequences, pairs of alignments, or a sequence and an alignment. This is useful in progressive multiple sequence alignment.

Fixes #550.
Fixed by forcing values in aligned_seqs1/aligned_seqs2 to be ``str`` above, which should also cut down on memory usage (their being ``BiologicalSequence`` objects was unnecessary in the intermediate data structure. Thanks for bringing my attention to this.