WarpedGP loses warp_tanh attributes when saved/loaded from disk

MWE:

```
X = np.random.randn(100, 2)
Y = np.random.randn(100, 1)
model = GPy.models.WarpedGP(X, Y)
# assert that it exists
assert hasattr(model, "warp_tanh")
model.optimize()
assert hasattr(model, "warp_tanh")
model.save_model("model")
model = GPy.models.WarpedGP.load_model("model.zip")
assert hasattr(model, "warp_tanh") # AssertionError
```