Clean wrapping of Knitro callbacks in Julia

[frapac]

This PR cleans the wrapping of Knitro callbacks. The wrappers are now using directly the information form the C objects rather than querying it from the Julia object KNITRO.Model.
Furthermore:

• we fix a bug in branch&bound, when using MINLPTests (in this case, Knitro copies the original model and adds new variables and constraints to handle new cuts)
• we now handle user interruption (CTRL+C) for all callbacks, not just evaluation callbacks
• we add missing methods to add primal and dual initial starts

Important notice: This PR adds a breaking change in the way user data are handled inside the callbacks. To fetch the user data, the user now has to write in the callbacks:

function eval_function(kc, cb, evalRequest, evalResult, userParams)
    my_data = userParams
    ...
end

instead of:

function eval_function(kc, cb, evalRequest, evalResult, userParams)
    my_data = userParams[:data]
    ...
end

Second important notice: This PR does not allow to resolve #93