Feature/node description (#325)

Refactor hardware-querying code.

Comprises part of the work on issue #318.

* Create `src/hardware` path for hardware-querying code.
* Create `nest::mc::hw` namespace for hardware-querying code.
* Move memory, affinity and power code from util and threading code to the new diretory and namespace.
* Add `nest::mc::threading::num_threads` function that attempts to determine the number of threads to use according to an environment variable, or by using the low level hardware querying if no environment variable is set.
* Add hardware query for counting the number of available GPUs.