Fixed mutable default value error in kernel dataclasses

Replaced direct assignments of kernels in VelocityKernel and HelmholtzKernel dataclasses with field(default_factory=...)
oceanmodelling example now works with python 3.11

docs/examples/oceanmodelling.py

@@ -10,7 +10,7 @@
 from jax import config
 
 config.update("jax_enable_x64", True)
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 
 from jax import hessian
 from jax import config
@@ -195,7 +195,6 @@ def dataset_3d(pos, vel):
 
 
 # %%
-from dataclasses import field
 
 
 @dataclass