Add GPU labels to template

Description of feature

Had discussion on slack with @ewels and Claus Hultschig whose Github Account I do not know.

Might not hurt to add some GPU labels that people can use. Note that for GPU support you need to either use a CUDA enabled base Docker container or add stuff yourself which still is kind of ugly.

Draft:

  withLabel: with_all_gpus {
       maxForks = 1
       containerOptions = {
          workflow.containerEngine == "singularity" ? '--nv':
              ( workflow.containerEngine == "docker" ? '-u $(id -u):$(id -g) --gpus all': null ) } 
  }

  withLabel: with_single_gpu {
    maxForks = 1
       containerOptions = {
          workflow.containerEngine == "singularity" ? '--nv':
              ( workflow.containerEngine == "docker" ? '-u $(id -u):$(id -g) --gpus device=0': null ) }
  } 
}

single-cpu needs the device to be parameterized.
Just opening this issue to keep track of this.

No file or test is named. Start by locating the template or configuration entry point that handles withLabel blocks, then review how Docker and Singularity container options are generated. Done means usable all-GPU and single-GPU labels are supported, with the single-GPU device parameterized as requested.