create model using python & (fix) exposure of joints

[fvalenza]

• The type of joint used as a root joint can now be specified by the user in C++ and in Python
• Add the possibility to create a model from scratch in python. To do so, first create an empty model using BuildEmptyModel(), then add one by one your joints with addBody(..args..)
• Also fixing a missing functionality : you now have properly access to the joints of the model.

Exemple

import pinocchio as se3

from pinocchio.romeo_wrapper import RomeoWrapper

myModel = se3.Model.BuildEmptyModel()

myModel.addBody(0, se3.JointModelFreeFlyer(), se3.SE3.Random(), se3.Inertia.Random(),"j1","b1",True)
myModel.addBody(1, se3.JointModelRX(), se3.SE3.Random(), se3.Inertia.Random(),"j2","b2",True)

joint =  myModel.joints[2]
print joint.idx_q
print joint.nq

[fvalenza]

@jcarpent new version of pull request.
parseTree and buildModel have been reworked to allow to be used with or without specifying a root joint. More user friendly.
Again: there are two methods in python to build a model because the macro BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS seems not be usable because of the different scopes