* Implementing a callback to switch between optimizers during training
* Implementing the R3Refinment for collocation points
* Modify trainer -> dataloader is created or updated by calling `_create_or_update_loader`
* Adding `add_points` routine to AbstractProblem so that new points can be added without resampling from scratch
* Solvers for multiple models
- Implementing the possibility to add multiple models for solvers (e.g. GAN)
- Implementing GAROM solver, see https://arxiv.org/abs/2305.15881
- Implementing tests for GAROM solver (cpu only)
- Fixing docs PINNs
- Creating a solver directory, for consistency in the package
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Co-authored-by: Dario Coscia <dariocoscia@dhcp-040.eduroam.sissa.it>
