Fix a bug where duplicated objectives can be passed

[tautomer]

Proposed fix for #95

    def suggest(self, **optim_kwargs):
        """
        Maps Optimizer.suggest method
        """
        if self.optimizer.is_fit:
            try:
                # In case X_space has changed, re-set the optimizer X_space
                self.optimizer.set_X_space(self.X_space)
                if "objective" not in optim_kwargs:
                    optim_kwargs["objective"] = self.objective
                X, eval = self.optimizer.suggest(
                    out_constraints=self.output_constraints,
                    **optim_kwargs,
                )
                return (X, eval)
            except Exception as e:
                warnings.warn('Optimization failed')
                print("Error message:", e)
                # print full traceback for debugging verbosity
                if self.optimizer.verbose > 2:
                    print("Stack trace:", traceback.format_exc())
                return None

class objective always takes priority and passed-in objective= is silently suppressed (might be bad for UX perspective. optionally raise a warning?)

Full traceback is printed out when verbosity > 2 (2 might be too high for critical messages like this).