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
@@ -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