Add layer to perform RBF interpolation in reduced order modeling (#315)

* add RBF implementation in pytorch (in layers)
* add POD-RBF example (baseline for nonintrusive closure)
* Add POD only and POD+RBF implementation
* add POD-RBF in tutorial 8

pina/model/layers/__init__.py

@@ -13,6 +13,7 @@ __all__ = [
     "FourierFeatureEmbedding",
     "AVNOBlock",
     "LowRankBlock",
+    "RBFBlock"
 ]
 
 from .convolution_2d import ContinuousConvBlock
@@ -27,3 +28,4 @@ from .pod import PODBlock
 from .embedding import PeriodicBoundaryEmbedding, FourierFeatureEmbedding
 from .avno_layer import AVNOBlock
 from .lowrank_layer import LowRankBlock
+from .rbf_layer import RBFBlock