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/__init__.py

@@ -8,6 +8,7 @@ __all__ = [
     "FourierIntegralKernel",
     "KernelNeuralOperator",
     "AveragingNeuralOperator",
+    "LowRankNeuralOperator"
 ]
 
 from .feed_forward import FeedForward, ResidualFeedForward
@@ -16,3 +17,4 @@ from .deeponet import DeepONet, MIONet
 from .fno import FNO, FourierIntegralKernel
 from .base_no import KernelNeuralOperator
 from .avno import AveragingNeuralOperator
+from .lno import LowRankNeuralOperator