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


---------

Co-authored-by: Dario Coscia <dariocoscia@dhcp-040.eduroam.sissa.it>

pina/solvers/__init__.py

@@ -0,0 +1,7 @@
+__all__ = [
+    'PINN',
+    'GAROM',
+]
+
+from .garom import GAROM
+from .pinn import PINN