Tests: Van Genuchten on the validity dataset

This test fits a VanGenuchten diffusivity to the frontx.examples.data.validity dataset using the neural API and checks the (reduced) chi-squared. It then validates by re-solving forward with the fitted diffusivity.

• Fit: frontx.neural.fit(D, o, theta, sigma=std, i=theta_i, b=theta_b)
• Stat: reduced chi-squared with ν = len(o) − 4

How to run

```bash pytest -q tests/test_vangenuchten.py