Modify pina.__init__.py and rm useless .py files

* rm meta.py, plotter.py, writer.py
* modify __init__ file
* modify tests due to __init__ import

Author: dario-coscia

pina/__init__.py

@@ -1,18 +1,15 @@
 __all__ = [
-    "Trainer", "LabelTensor", "Plotter", "Condition",
-    "PinaDataModule", 'TorchOptimizer', 'Graph',
-    "RadiusGraph", "KNNGraph"
+    "Trainer",
+    "LabelTensor", 
+    "Condition",
+    "PinaDataModule",
+    'Graph',
+    "SolverInterface",
+    "MultiSolverInterface"
 ]
 
-from .meta import *
 from .label_tensor import LabelTensor
-from .solvers.solver import SolverInterface
+from .graph import Graph
+from .solvers.solver import SolverInterface, MultiSolverInterface
 from .trainer import Trainer
-from .plotter import Plotter
 from .condition.condition import Condition
-
-from .data import PinaDataModule
-
-from .optim import TorchOptimizer
-from .optim import TorchScheduler
-from .graph import Graph, RadiusGraph, KNNGraph