Hessian preconditioner wrappers

[smartalecH]

Closes #177

There's still an issue with the python wrappers. Seems to be passing the wrong number of arguments:

NotImplementedError: Wrong number or type of arguments for overloaded function 'opt_set_precond_min_objective'.
  Possible C/C++ prototypes are:
    nlopt::opt::set_precond_min_objective(nlopt::func,nlopt::pfunc,void *)
    nlopt::opt::set_precond_min_objective(nlopt::func,nlopt::pfunc,void *,nlopt_munge,nlopt_munge)

Any ideas what might be wrong?

[jschueller]

hello @smartalecH, any news on this ?