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

tests/test_problem_zoo/test_supervised_problem.py

@@ -2,7 +2,7 @@ import torch
 from pina.problem import AbstractProblem
 from pina.condition import InputOutputPointsCondition
 from pina.problem.zoo.supervised_problem import SupervisedProblem
-from pina import RadiusGraph
+from pina.graph import RadiusGraph
 
 def test_constructor():
     input_ = torch.rand((100,10))