Fix bindings for contact-6d

Currently I receive following as the docstring of ContactModel6d in python

In [1]: from crocoddyl import ContactModel6D

In [2]: ContactModel6D?
Docstring:     
Rigid 6D contact model.

It defines a rigid 6D contact models based on acceleration-based holonomic constraints.
The calc and calcDiff functions compute the contact Jacobian and drift (holonomic constraint) or
the derivatives of the holonomic constraint, respectively.
Init docstring:
__init__( (object) self, (StateMultibody) state, (FramePlacement) Mref, (int) nu=state.nv [, (object) gains=np.matrix([ [0.],[0.] ]]) -> None :
    Initialize the contact model.
    
    :param state: state of the multibody system
    :param Mref: reference frame placement
    :param nu: dimension of control vector
    :param gains: gains of the contact model

    C++ signature :
        void __init__(_object*,boost::shared_ptr<crocoddyl::StateMultibody>,crocoddyl::FramePlacement,int [,Eigen::Matrix<double, 2, 1, 0, 2, 1>])

__init__( (object)arg1, (StateMultibody) self, (FramePlacement) state [, (object) Mref]) -> None :
    Initialize the state cost model.
    
    :param state: state of the multibody system
    :param Mref: reference frame placement
    :param nu = dimension of control vector

    C++ signature :
        void __init__(_object*,boost::shared_ptr<crocoddyl::StateMultibody>,crocoddyl::FramePlacement [,Eigen::Matrix<double, 2, 1, 0, 2, 1>])

If you see:

1. The second init mentions state cost model.
2. The argument list for the second init model is (StateMultibody) self, (FramePlacement) state [, (object) Mref]). This is not right.