fix: fix number of download threads reportedly being 1 on some machines regardless of the configuration

[leoisl]

This PR fixes number of download threads reportedly being 1 on some machines regardless of the configuration.
It worked on EBI slurm cluster, please test in your before merging.

Closes #242

Note 1: I had to remove -p priority when submitting the slurm job as this queue does not exist in EBI slurm cluster so I think is not standard;
Note 2: why not tell snakemake to use the user's slurm profile and let it deal with it, like we do for the lsf profile?

[karel-brinda]

Thank you very much for the fix!!

Re Note 2: We should discuss this in a separate issue. I have never used slurm profiles so I'm unable to tell anything. The slurm command was mainly an adhoc thing for the Harvard O2 cluster (and this cluster requires a queue specification).