Miniwdl-slurm

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+ Add support for ``slurm_partition`` runtime attribute.
+ Adds support for ``gpu`` and ``gpuCount`` runtime attributes. The ``gpu`` runtime
attribute is a boolean that indicates whether the task requires a GPU. The
``gpuCount`` runtime attribute is an integer that indicates the number of GPUs
required by the task.
+ Add support for ``slurm_constraint`` runtime attribute.

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Initial release with the following features:

+ Utilize miniwdl's singularity backend to create a singularity command that
is then submitted using srun.
+ Create a singularity image cache so singularity images are available on
the cluster nodes.
+ Support for ``cpu``, ``memory`` and ``time_minutes`` runtime attributes.