STK_MINIMIZE_UNCONSTRAINED performs unconstrained minimization

 CALL: U_OPT  = stk_minimize_unconstrained (ALGO, F, U_INIT)

   minimizes the objective function F, using algorithm ALGO with starting
   point U_INIT. The best point found by the algorithm is returned as U_OPT.
   The value of ALGO can be 'fminsearch', 'octavesqp', or any algorithm object
   implementing the 'stk_minimize_boxconstrained' method.

 CALL: [U_OPT, F_OPT] = stk_minimize_unconstrained (ALGO, F, U_INIT)

   also returns the best objective value F_OPT.

 See also stk_minimize_boxconstrained, stk_optim_octavesqp, stk_optim_fminsearch