Low Rank Neural Operator (#270)

* add the Low Rank Neural Operator as Model
* add the Low Rank Layer as Layer
* adding tests
* adding doc 

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Co-authored-by: Dario Coscia <dariocoscia@Dario-Coscia.local>
Co-authored-by: Nicola Demo <demo.nicola@gmail.com>

pina/model/layers/__init__.py

@@ -11,6 +11,7 @@ __all__ = [
     "PODBlock",
     "PeriodicBoundaryEmbedding",
     "AVNOBlock",
+    "LowRankBlock",
     "AdaptiveActivationFunction",
 ]
 
@@ -25,4 +26,5 @@ from .fourier import FourierBlock1D, FourierBlock2D, FourierBlock3D
 from .pod import PODBlock
 from .embedding import PeriodicBoundaryEmbedding
 from .avno_layer import AVNOBlock
-from .adaptive_func import AdaptiveActivationFunction
+from .lowrank_layer import LowRankBlock
+from .adaptive_func import AdaptiveActivationFunction