Downsample

These are the commits that implement downsampling when importing instrument data.
I made [this sketch](https://gist.github.com/mark-burnett/64e92ab542a0cc6447a6) to demonstrate an approach to using `Genome::WorkflowBuilder` to construct the import workflows.  It's definitely missing some important features that the current importer has, but I think it shows how it might help make the code a bit more readable (though maybe that's a matter of opinion).

The sketch also allows importing multiple types of files simultaneously, though I'm not sure that's a valuable feature.  You could additionally specify different LSF resource parameters for each file (assuming you're running on Flow/PTero) using WorkflowBuilder, which I don't think you can easily do without it.