Missing core features

[bchretien]

Some important features are currently missing:

• get/set solver parameters (e.g. ipopt.output_file) (b2c2217)
• access elements of solver result (04bf70a)
• access warnings in result (05fe4ee)
• finite differences support (fc04ea2)
• iteration callback support (a3111c5)
• Jacobian callback support (80480f9)
• optimization logger support (95bf83c)
• solver callback multiplexer support (93046db)
• cached function support (0f82ad4)
• storage order support (4fd71b3)
• sparse function support
• hessian computation

Known bugs:

• differentiable functions printed as non differentiable (f2e93e9)
• adding constraints leads to segv (8d6143d)

(complete this list as needed)