Solvers for multiple models (#133)

* 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>

pina/trainer.py

@@ -3,7 +3,7 @@
 import lightning.pytorch as pl
 from .utils import check_consistency
 from .dataset import DummyLoader
-from .solver import SolverInterface
+from .solvers.solver import SolverInterface
 
 class Trainer(pl.Trainer):