velvet: add OMP_THREAD_LIMIT

[bernt-matthias]

if not set the roadmap files (and assemblies?) differ when using different numbers of cores

see also https://www.biostars.org/p/86907/

.. does not work for 1 CPU https://github.com/dzerbino/velvet/issues/56

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[bernt-matthias]

Yes, still a draft. Trying to figure out upstream how to fix this correctly. See linked issue.