Muscodwalkin. Closes #72

Loads references from the centroidal solver, and runs ddp to track those references.

Some work still needs to be done to define properly the tasks (e.g., to add high cost velocity tracking on foot during contact transitions).

The current framework is meant to deal with multi-contact locomotion (and not just bipedal walking)

crocoddyl/__init__.py

+from state import StateAbstract
 from state import StateVector, StateNumDiff
 from state import StatePinocchio
 from cost import CostDataPinocchio, CostModelPinocchio
@@ -18,7 +19,7 @@ from action import ActionDataLQR, ActionModelLQR
 from action import ActionDataNumDiff, ActionModelNumDiff
 from integrated_action import IntegratedActionDataEuler, IntegratedActionModelEuler
 from integrated_action import IntegratedActionDataRK4, IntegratedActionModelRK4
-from differential_action import DifferentialActionData, DifferentialActionModel
+from differential_action import DifferentialActionDataFullyActuated, DifferentialActionModelFullyActuated
 from differential_action import DifferentialActionDataLQR, DifferentialActionModelLQR
 from differential_action import DifferentialActionDataNumDiff, DifferentialActionModelNumDiff
 from floating_contact import DifferentialActionDataFloatingInContact, DifferentialActionModelFloatingInContact
@@ -39,5 +40,5 @@ from callbacks import CallbackDDPLogger, CallbackDDPVerbose, CallbackSolverDispl
 from ddp import SolverDDP
 from kkt import SolverKKT
 from robots import getTalosPathFromRos, loadTalosArm, loadTalos, loadTalosLegs, loadHyQ
-from utils import m2a, a2m, absmax, absmin, EmptyClass
+from utils import m2a, a2m, absmax, absmin
 from diagnostic import plotDDPConvergence, plotOCSolution, displayTrajectory