MP2NL meta-driver, Gurobi's signpow()

What's Changed

• MP2NL meta-driver: https://mp.ampl.com/modeling-tools.html#meta-driver-mp2nl
• Support tanh() and signpow() for Gurobi 13: https://mp.ampl.com/modeling-expressions.html#general-nonlinear-functions
• fix URL for nlwrite.pdf by @svigerske in #271

Full Changelog: v4.0.4...v4.1.0