Add layer to perform RBF interpolation in reduced order modeling

[annaivagnes]

Added the RBFLayer class in the pina/model/layers folder, and updated tutorial8 with a comparison between POD-RBF and POD-NN approaches in reduced order modeling framework.

[ndem0]

Fine for me, we can merge @dario-coscia

[dario-coscia]

Hi @annaivagnes ! Thanks for the PR updates. I have a minor question: does the layer work correctly on a different device than cpu? Also, it would be great to add pytests for the layer, like in the pod tests. After test addition for me it can also be merged

[annaivagnes]

@dario-coscia thanks for the suggestions, I changed RBFLayer into RBFBlock and added some tests. It also works with GPU!

[dario-coscia]

Great! Thanks @annaivagnes this addition looks useful🚀🚀 @ndem0 we can merge for me