Add grid search and plot to SimpleNN and GroupsNN

[AgathaSchmidt]

Motivation / Current Behaviour

Right now, we only provide the data gernation as well as the final neural network based model for the surrogate models for one population in the main branch.

Enhancement description

Now we decided that we want to provide the code for finding the final architecture via a grid search etc. and the code for the visualizations.

• [ ] grid search (layers, units per layer)
• [ ] hyperparameter testing
• [ ] plots

Additional context

No response

Checklist

• [X] Attached labels, especially loc:: or model:: labels.
• [X] Linked to project