Gpu/restrict all the things (#1026)

Make all pointer arguments to kernels `__restrict__` to avoid unnecessary loads.

The effect on the busyring benchmark (swapped pas -> hh) with 8192 cells on a V100 GPU (time for model-run in seconds): 
```
|----------+-------|
| Baseline | After |
|----------+-------|
|    2.347 | 2.268 |
|    2.345 | 2.262 |
|    2.321 | 2.276 |
|    2.323 | 2.267 |
|    2.330 | 2.249 |
|----------+-------|
|    2.321 | 2.249 |
|----------+-------|
```