allow merged downloads in `gbsketch`

[bluegenes]

b/c API key more simultaneous downloads than urlsketch

could allow ';'-separated input in the acc column. NCBI API allows download of multiple accessions in the same zipfile, but we would need to modify GBAssemblyData /input

[bluegenes]

we now use AssemblyData and share processing code across both gbsketch and urlsketch. We would need to modify accession loading and attaching URLs to allow this?