move optimizer into controllers folder

guided_mrmp/controllers/mpc.py

 import numpy as np
 import casadi as ca
-from guided_mrmp.optimizer import Optimizer
+from guided_mrmp.controllers.optimizer import Optimizer
 
 class MPC:
     def __init__(self, model, T, DT, state_cost, final_state_cost, input_cost, input_rate_cost, settings):

guided_mrmp/controllers/multi_mpc.py

 import numpy as np
 import casadi as ca
-from guided_mrmp.optimizer import Optimizer
+from guided_mrmp.controllers.optimizer import Optimizer
 
 np.seterr(divide="ignore", invalid="ignore")