Fix get chain

[Giulero]

In methods like forward_kinematics or jacobian, we need to access the kinematic tree that goes from the base frame to the end-effector. To get this kinematic chain I use the method get_chain (from urdf_parser_py), which returns the sequence of the kinematic tree as [...-link-joint-link-joint-...]
Sometimes a link and a joint may have the same name.

Before I was just checking that an element in the chain is in the joints list, supposing that links and joints could not have the same name. For example, if a joint and a link are named right_elbow, the check would return that both the elements are in the joints list.

To solve this issue I'm allowing the method get_chain to return only the joints in the chain.

Changing:

chain = self.robot_desc.get_chain(self.root_link, frame)
...
if item in self.robot_desc.joint_map:

with

chain = self.robot_desc.get_chain(self.root_link, frame, links=False)

I'm using this PR also to allow the creation of a link list with only links with inertial properties. With this modification, no fictitious zero inertia is needed. (See #23 (comment))

[Giulero]

Thanks @FabioBergonti!