added: fetching actual LangrangeMultiplier in getinfo

[franckgaga]

For optimizer that supports it, the Hessian of the Lagrangians returned by getinfo function, that is:

• :∇²ℓg or :nabla2lg fields in info dictionnary
• :∇²ℓgeq or :nabla2lgeq fields in info dictionnary

will now contain the actual second order derivatives at the optimal Lagrange multipliers and decision variables.

If the optimizer does not support fetching the Lagrange multipliers, the non-zero coefficients in the matrices will be random values, since just the sparsity structure can be useful in itself (with a @warn message)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.55%. Comparing base (399968e) to head (2a1cc13).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #298   +/-   ##
=======================================
  Coverage   98.55%   98.55%           
=======================================
  Files          28       28           
  Lines        4967     4980   +13     
=======================================
+ Hits         4895     4908   +13     
  Misses         72       72           

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