Support nonlinear constraints with Gurobi 12

[simonbowly]

The upcoming Gurobi 12 release supports nonlinear constraints through gurobipy's NLExpr class. There are a few steps required to integrate it in gurobipy-pandas:

• Recognise NLExpr's in constraints, and pass them to addGenConstrNL (the arithmetic is taken care of, e.g. x**4 will "just work" on pandas series)
• Add some API for applying nonlinear functions to expressions, e.g. a .gppd.exp accessor or gppd.exp would map to nlfunc.exp For now, document .apply as the appropriate method for applying nlfuncs.
• Consider whether to automatically add intermediate variables to allow more flexibility in constraint senses & objectives For now, keep the same restrictions as gurobipy and document the recommended approach to intermediate variables.