[epic] Optimise efficiency

[mastoffel]

There are several bits where efficiency can be improved:

• [ ] Cross-validation: We currently run cross-validation after hyperparameter optimisation. This isn't strictly necessary, as the optimisation with RandomizedSearchCV and BayesSearchCV do already contain the relevant cross-validations. However, their output is less detailed than cross_validate and doesn't contain the results for each cv, but only their averages. The question is: Is there a way to not run cross_validate again after grid search?
• [ ] Parallelisation: Some models, such as LightGBM, take an n_jobs argument. Currently, these are always set to 1, so that we only parallelise using cross_validate or grid search, but not within models. Is that the best way?
• [ ] Does autoemulate run well on a cluster?
• [ ] GPU support?

[kallewesterling]

@mastoffel I have tried to break down this epic into more bite-sized chunks, but there are obviously things missing (see the "..." above) so feel free to edit this. Then we can make these into issues today!