Quick fixes

[wz-ml]

Changes:

• [x] Add temperature as an optional (deprecated) param to the SGLD sampler.
• [x] Add support for separate seeds for SGLD noise and the dataloader, as per Aether PR #41.
• [x] Add GPU - CPU LLC comparison test.
• [x] Fix multi-core sampling (multiprocessing on CPUs). Issue raised by Naoki Hayashi.
• [x] Integrate MalaAcceptanceRate with Aether.
• [ ] Add Multi-GPU sampling support (list of GPU devices?)
  • [x] Support multiprocessing with a single GPU (provides speedups if GPU has spare capacity).
• [x] Rename optimal_nbeta $\rightarrow$ default_nbeta.
• [x] Fix for Issue #88
• [x] Fix for Issue #90

All changes should be immediately mergeable with no impact on existing functionality.