allow merging multiple files into single sketch?

[bluegenes]

= create merged sketches directly.

Use cases:

• read1/read2
• sequences from virus segments that we'd like in a single sketch

To add, we would need to enable downloading + sketching a list of accessions/urls:

1. modify csv reading to allow merged format (';'-separated list of urls/accessions?)
2. add InputData struct or similar that could hold multiple AccessionData/GBAssemblyData? or just use vec
3. make dl_sketch_merged or modify dl_sketch_assembly_accession to loop