Fix Codacy Warnings (#477)

---------

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

pina/problem/spatial_problem.py

@@ -13,27 +13,7 @@ class SpatialProblem(AbstractProblem):
     Here's an example of a spatial 1-dimensional ODE problem.
 
     :Example:
-        >>> from pina.problem import SpatialProblem
-        >>> from pina.operator import grad
-        >>> from pina.equation import Equation, FixedValue
-        >>> from pina import Condition
-        >>> from pina.geometry import CartesianDomain
-        >>> import torch
-        >>>
-        >>>
-        >>> class SpatialODE(SpatialProblem:
-        >>>
-        >>>     output_variables = ['u']
-        >>>     spatial_domain = CartesianDomain({'x': [0, 1]})
-        >>>
-        >>>     def ode_equation(input_, output_):
-        >>>         u_x = grad(output_, input_, components=['u'], d=['x'])
-        >>>         u = output_.extract(['u'])
-        >>>         return u_x - u
-        >>>
-        >>>     conditions = {
-        >>>         'x0': Condition(CartesianDomain({'x': 0, 'alpha':[1, 10]}), FixedValue(1.)),
-        >>>         'D': Condition(CartesianDomain({'x': [0, 1], 'alpha':[1, 10]}), Equation(ode_equation))}
+        TODO
     """
 
     @abstractmethod
@@ -41,7 +21,6 @@ class SpatialProblem(AbstractProblem):
         """
         The spatial domain of the problem.
         """
-        pass
 
     @property
     def spatial_variables(self):