Separate node and partition configuration

[sjpb]

This repeats #183 which was reverted in ed717da to allow #186 to be merged.

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Replaces openhpc_slurm_partitions with openhpc_nodegroups and openhpc_partitions, to make complex partition configurations correct. Previously it would have been possible to define nodes twice, with different parameters, and only the first occurance would have been used.

As part of these changes this:

• Adds nodegroup.node_params to allow specifying additional Node-level parameters
• Adds nodegroup.features to allow specifying additional Node features
• Removes openhpc_slurm_partitions.cluster_name
• Removes openhpc_slurm_partitions.extra_nodes