Support gpus on slurm

[williamrowell]

• added runtime variable slurm_partition (string)
• added runtime variable num_gpu (int)
• if gpu is true, max(1,num_gpu) gpus will be requested
• added runtime variable constraint (string)

Checklist

• [x] Pull request details were added to CHANGELOG.rst

I can change num_gpu to gpuCount if preferred.

I've tested all of these internally and can build a test if that would help.

[codecov-commenter]

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[rhpvorderman]

Release is done! https://pypi.org/project/miniwdl-slurm/0.2.0/

Thanks again @williamrowell !