Improve conditions and refactor dataset classes (#475)

* Reimplement conditions

* Refactor datasets and implement LabelBatch

---------

Co-authored-by: Dario Coscia <dariocos99@gmail.com>

tests/test_solver/test_supervised_solver.py

@@ -1,7 +1,7 @@
 import torch
 import pytest
 from pina import Condition, LabelTensor
-from pina.condition import InputOutputPointsCondition
+from pina.condition import InputTargetCondition
 from pina.problem import AbstractProblem
 from pina.solver import SupervisedSolver
 from pina.model import FeedForward
@@ -14,8 +14,8 @@ class LabelTensorProblem(AbstractProblem):
     output_variables = ['u']
     conditions = {
         'data': Condition(
-            input_points=LabelTensor(torch.randn(20, 2), ['u_0', 'u_1']),
-            output_points=LabelTensor(torch.randn(20, 1), ['u'])),
+            input=LabelTensor(torch.randn(20, 2), ['u_0', 'u_1']),
+            target=LabelTensor(torch.randn(20, 1), ['u'])),
     }
 
 
@@ -24,8 +24,8 @@ class TensorProblem(AbstractProblem):
     output_variables = ['u']
     conditions = {
         'data': Condition(
-            input_points=torch.randn(20, 2),
-            output_points=torch.randn(20, 1))
+            input=torch.randn(20, 2),
+            target=torch.randn(20, 1))
     }
 
 
@@ -36,7 +36,7 @@ def test_constructor():
     SupervisedSolver(problem=TensorProblem(), model=model)
     SupervisedSolver(problem=LabelTensorProblem(), model=model)
     assert SupervisedSolver.accepted_conditions_types == (
-        InputOutputPointsCondition
+        InputTargetCondition
     )