Refactor hardware detection to sup (#654)

Refactoring that moves the logic for determining available concurrency and available GPUs from the core Arbor library to the sup library. This also constitutes work towards providing functionality for allocating GPUs to particular ranks when multiple GPUs are visible per rank.

* Move core/thread estimation code to sup library.
* Change default resource behaviour to use one thread and no GPU.
* Provide an interface in the sup library for: acquiring a default GPU; for coordinating an allocation of GPUs across multiple MPI ranks.