Implementation of DataLoader and DataModule (#383)

Refactoring for 0.2
* Data module, data loader and dataset
* Refactor LabelTensor
* Refactor solvers

Co-authored-by: dario-coscia <dariocos99@gmail.com>

pina/problem/abstract_problem.py

@@ -90,10 +90,9 @@ class AbstractProblem(metaclass=ABCMeta):
             variables += self.spatial_variables
         if hasattr(self, "temporal_variable"):
             variables += self.temporal_variable
-        if hasattr(self, "unknown_parameters"):
+        if hasattr(self, "parameters"):
             variables += self.parameters
-        if hasattr(self, "custom_variables"):
-            variables += self.custom_variables
+
 
         return variables
 
@@ -170,7 +169,6 @@ class AbstractProblem(metaclass=ABCMeta):
                     f"Wrong variables for sampling. Variables ",
                     f"should be in {self.input_variables}.",
                 )
-
         # check correct location
         if locations == "all":
             locations = [