Documentation for v0.1 version (#199)

* Adding Equations, solving typos
* improve _code.rst
* the team rst and restuctore index.rst
* fixing errors

---------

Co-authored-by: Dario Coscia <dariocoscia@dhcp-015.eduroam.sissa.it>

pina/problem/spatial_problem.py

@@ -14,24 +14,25 @@ class SpatialProblem(AbstractProblem):
     :Example:
         >>> from pina.problem import SpatialProblem
         >>> from pina.operators import grad
-        >>> from pina import Condition, Span
+        >>> from pina.equations import Equation, FixedValue
+        >>> from pina import Condition
+        >>> from pina.geometry import CartesianDomain
         >>> import torch
-        >>> class SimpleODE(SpatialProblem):
+        >>>
+        >>>
+        >>> class SpatialODE(SpatialProblem:
+        >>>
         >>>     output_variables = ['u']
-        >>>     spatial_domain = Span({'x': [0, 1]})
+        >>>     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
-        >>> 
-        >>>     def initial_condition(input_, output_):
-        >>>         value = 1.0
-        >>>         u = output_.extract(['u'])
-        >>>         return u - value
         >>>
         >>>     conditions = {
-        >>>         'x0': Condition(Span({'x': 0.}), initial_condition),
-        >>>         'D': Condition(Span({'x': [0, 1]}), ode_equation)}
+        >>>         'x0': Condition(CartesianDomain({'x': 0, 'alpha':[1, 10]}), FixedValue(1.)),
+        >>>         'D': Condition(CartesianDomain({'x': [0, 1], 'alpha':[1, 10]}), Equation(ode_equation))}
 
     """