Neural Tangent Kernel integration + typo fix (#505)

* NTK weighting + typo fixing
* black code formatter + .rst docs

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Co-authored-by: Dario Coscia <dariocos99@gmail.com>

pina/loss/__init__.py

@@ -6,6 +6,7 @@ __all__ = [
     "PowerLoss",
     "WeightingInterface",
     "ScalarWeighting",
+    "NeuralTangentKernelWeighting",
 ]
 
 from .loss_interface import LossInterface
@@ -13,3 +14,4 @@ from .power_loss import PowerLoss
 from .lp_loss import LpLoss
 from .weighting_interface import WeightingInterface
 from .scalar_weighting import ScalarWeighting
+from .ntk_weighting import NeuralTangentKernelWeighting