* Change the behavior of MakeParameter, depending on the callback:
    - `SparseCallback`: remove the fixed variables and reduce problem's dimension in MadNLP
    - `DenseCallback`: keep the fixed variables frozen (as done previously)
* Add a new `fixed_handler` if the problem has no fixed variable: `NoFixedVariable`.
* Ensure we always interact to the nonlinear program `nlp` through the
  callback wrapper `cb` to avoid side effect.
* Remove the function `get_index_constraint` and stores indexes directly
  in the callback wrapper `cb`.
* Fix the computation of the bound's multipliers for fixed variables.