Consider mechanism to specify multiple tasks per core

[hategan]

Currently, the spec supports binding processes to cores using job.spec.resources.cpu_cores_per_process, which is assumed to be an integer. However, this does not allow one to specify jobs that can put multiple processes per core, as the --ntasks-per-core SLURM option would allow. We should consider a mechanism to allow either multiple cores per task or multiple tasks per core, such as specifying a task-to-core ratio, in the form n/m.