Diff information of dual variables

[matbesancon]

Following a discussion on #91 it might be interesting to add the attributes for dual variables

[joaquimg]

The case for NLP is done in #260

[joaquimg]

replicating the message on #315

As pointed out in #303 by @blegat ,
we can also implement Dual Objective Sensitivity wrt parameters using the gradients of the dual objective and the dual variable sensitivity.

Implement:

• ForwardDualObjectiveSensitivity for NonLinearProgram;
• ReverseDualObjectiveSensitivity for NonLinearProgram;
• ForwardDualObjectiveSensitivity for ConicProgram;
• ReverseDualObjectiveSensitivity for ConicProgram;
• ForwardDualObjectiveSensitivity for QuadraticProgram;
• ReverseDualObjectiveSensitivity for QuadraticProgram;