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/__init__.py

@@ -10,7 +10,7 @@ __all__ = [
 
 from .meta import *
 from .label_tensor import LabelTensor
-from .pinn import PINN
+from .solvers.pinn import PINN
 from .trainer import Trainer
 from .plotter import Plotter
 from .condition import Condition