experiment with converting quadratic functions to nonlinear, to simplify/streamline code

Pavito is harder to maintain, debug, and read/understand while the quadratic functions are handled separately from the more general nonlinear functions.

after ScalarNonlinearFunction is finalized and https://github.com/jump-dev/MathOptInterface.jl/pull/2059 is in, I think we should think about perhaps a bridge from ScalarQuadraticFunction to ScalarNonlinearFunction in MOI, or if not a bridge then a more manual conversion inside Pavito.

@odow any thoughts? I'd like to have a shot at this when the time is right.