Add Averaging Neural Operator with tests and a tutorial (#230)

* add Averaging Neural Operator with tests
* add backward test
* minor changes
* doc addition

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Co-authored-by: Dario Coscia <dariocoscia@Dario-Coscia.local>

pina/model/layers/__init__.py

@@ -10,6 +10,7 @@ __all__ = [
     "FourierBlock3D",
     "PODBlock",
     "PeriodicBoundaryEmbedding",
+    "AVNOBlock",
 ]
 
 from .convolution_2d import ContinuousConvBlock
@@ -22,3 +23,4 @@ from .spectral import (
 from .fourier import FourierBlock1D, FourierBlock2D, FourierBlock3D
 from .pod import PODBlock
 from .embedding import PeriodicBoundaryEmbedding
+from .avno_layer import AVNOBlock