In operaFEA, i can define the geometry parameters as model demensions in user variables. And i can use the optimizer block to find the best geometry parameters. But if i want to find the best boundary conditions, what should i do？For example, i want to simulate the electron gun model in operaFEA and find the best focal electrode voltage(boundary conditions)  to make the electron focal spot min, how can i find the best voltage on the focal electrode.