find a safe way to set states/pos/velocity

[sbarthelemy]

currently, a user can mess everything when setting a world state.

For instance joint.gvel[0] = 2. is ok, but joint.gvel = 2. is not, as it breaks the link between joint.gvel and world.gvel. The same problem occurs with joint.gpos.

maybe joint.gvel should be removed completely, as it is redundant with world.gvel. Currently, joint.gvel is used...

• in joint.integrate(), but it could be passed as an argument to integrate.
• to set the joint initial conditions. This could be done with world.gvel[joint.dof] = ...
• in joint.twist, which is itself called in Body.update_dynamic(). It's less easy to remove it from there.

Note also that we do not povide a joint.gforce interface, which could be useful and seem logical as long as there is joint.gvel.

The JointsList class is an attemps to provide a nicer interface. It currently provides

• joints.dof (a 1d array or a slice)
• joints.gvel (a 1d array wich is a copy of the selected joints gvels)
• joints.gpos (a 1d array wich is a copy of the selected joints gpos if they are 1d)

maybe the gvel and gpos should be dropped, or moved in another class (such as a per-robot one: human36, ...)).

For reference, KDL has a different approach: they have a single joint class that handles all the (1dof) joints, and which does not store the joint state at all.