Issue on page /test-functions/piston.html

[AdelaHlobilova]

Thank you for the helpful library of UQ test functions!

I noticed that the expression for the volume $V$ includes a nested square root
https://uqtestfuns.readthedocs.io/en/latest/test-functions/piston.html#test-functions-piston

However, in the original publication:
Ben-Ari, E. N., & Steinberg, D. M. (2007). Modeling Data from Computer Experiments: An Empirical Comparison of Kriging with MARS and Projection Pursuit Regression. Quality Engineering, 19(4), 327–338.
https://doi.org/10.1080/08982110701580930

The formula contains only a single square root:
$$V = \frac{S}{2k} \left( \sqrt{A^2 + 4k \frac{P_0 V_0}{T_0} T} - A \right)$$

Would you consider reviewing this part of the documentation?

Thank you again for your work on this project.