Change JointPtr_t to avoid malloc

[jmirabel]

Currently, JointPtr_t is boost::shared_ptr<Joint>. I think it could simply be something like:

class JointPtr_t
{
DevicePtr_t d;
JointIndex jid;

Joint& operator-> ();
Joint& operator* ();
};

[nmansard]

The smart pointers design is inherited from hpp-model for all objects except for joints. We inserted smart pointers for joints to enforce symmetry in the design. I still think that this symmetry objective is a good choice. Why did you change your mind?

[jmirabel]

JointPtr_t used to be a Joint* and not a shared pointer but that's not my reason.

1. When updating hpp-core to use hpp-pinocchio, I had a loop which was creating many JointPtr_t from new Joint implicitly. The result was that the whole code was really slow. It didn't take me long to know where the issue was because I know what functions allocate new Joint. However, this behaviour might be surprising for people not having such knowledge.
2. IMHO, there should not be a malloc when no actual object is created.