Parallelization of get_pairwise_alignments

[danpal96]

All pytest tests passed and outputs are identical (except for the order of the calculations of the pairwise alignments in the log) in a test with contigs

Issue #63

[rrwick]

Thanks for this - I've merged it! Just do keep the log consistent and tidy, I've made a few tweaks (d987636) to ensure that the pairwise alignment results are printed in order.