Tutorials and Doc (#191)

* Tutorial doc update
* update doc tutorial
* doc not compiling

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Co-authored-by: Dario Coscia <dcoscia@euclide.maths.sissa.it>
Co-authored-by: Dario Coscia <dariocoscia@Dario-Coscia.local>

docs/source/_rst/_code.rst

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+Code Documentation
+==================
+Welcome to PINA documentation! Here you can find the modules of the package divided in different sections.
+
+PINA Features
+-------
+.. toctree::
+    :titlesonly:
+
+    LabelTensor <label_tensor.rst>
+    Condition <condition.rst>
+    Plotter <plotter.rst>
+
+Problem
+-------
+
+.. toctree::
+    :titlesonly:
+
+    AbstractProblem <problem/abstractproblem.rst>
+    SpatialProblem <problem/spatialproblem.rst>
+    TimeDependentProblem <problem/timedepproblem.rst>
+    ParametricProblem <problem/parametricproblem.rst>
+
+Solvers
+-------
+
+.. toctree::
+    :titlesonly:
+    
+    SolverInterface <solvers/solver_interface.rst>
+    PINN <solvers/pinn.rst>
+
+
+Models
+-----
+
+.. toctree::
+    :titlesonly:
+
+    Network <models/network.rst>
+    FeedForward <models/fnn.rst>
+    MultiFeedForward <models/multifeedforward.rst>
+    ResidualFeedForward <models/fnn_residual.rst>
+    DeepONet <models/deeponet.rst>
+    FNO <models/fno.rst>
+
+Layers
+------
+
+.. toctree::
+    :titlesonly:
+
+    ContinuousConv <layers/convolution.rst>
+    
+
+Geometries
+----------
+
+.. toctree::
+    :titlesonly:
+
+    Location <geometry/location.rst>
+    CartesianDomain <geometry/cartesian.rst>
+    EllipsoidDomain <geometry/ellipsoid.rst>
+    SimplexDomain <geometry/simplex.rst>
+
+
+Loss
+------
+
+.. toctree::
+    :titlesonly:
+
+    LossInterface <loss/loss_interface.rst>
+    LpLoss <loss/lploss.rst>
+    PowerLoss <loss/powerloss.rst>