Adding example of hybrid simulation with DNN for rel perm

[jakobtorben]

Added an example demonstrating how to integrate neural networks into JutulDarcy. Demonstrated with a simple example that uses a neural network trained to match the Brooks Corey analytical function.

The example includes:

1. Set up a simple reference simulation with a Brooks-Corey relative permeability model
2. Train a neural network to approximate the Brooks-Corey relative permeability model
3. Incorporate the trained network into a simulation model
4. Compare the results of the neural network-based simulation with the reference simulation