Enh: multi-sample workflow

[pvanheus]

Users often process multiple samples at once. When it comes to running pangolin etc it makes sense to run a single batch of samples because then you only update the DB once. Also you can imagine a single "workflow report" at the end, especially if we get the HTML output fixed.

This would have to loop and update the metadata for each sample. Also its not clear exactly how to present e.g. a collection of FASTA or a collection of VCF or TSV as output.