Example: Validity of capillary imbibition models

This example reproduces the comparison from:

Gerlero, G. S., Valdez, A. R., Urteaga, R., & Kler, P. A. (2022).
Validity of capillary imbibition models in paper-based microfluidic applications.
Transport in Porous Media, 141(2), 359–378. https://doi.org/10.1007/s11242-021-01724-w

It compares four models (BrooksAndCorey, VanGenuchten, LETxs, LETd) against experimental data using the public API frontx.solve.

How to run

From the command line: ```bash python -m frontx.examples.validity