* Unify integral kernel NO architectures with NeuralKernelOperator
* Implement FNO based on NeuralKernelOperator
* modify doc for FNO and add for FourierIntegralKernel, NeuralKernelOperator
* adding tests
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Co-authored-by: Dario Coscia <dariocoscia@Dario-Coscia.local>
Co-authored-by: Dario Coscia <dariocoscia@dhcp-015.eduroam.sissa.it>
