Output set of assemblies

[Casey Dunn]

One of the most valuable ways to represent assembly results is as a set of assemblies drawn from the posterior. This can be propogated to downstream analyses. We should add a feature to export the sequences of posterior assemblies, either as a single fasta where each sequence is one assembly (would only work when a single contig is produced, or as a folder of fastas with one assembly per fasta file.

Full posterior is huge, so we could output every nth assembly. n=100 would be a reasonable default.