Improving Jacobian laboratory

The main change is some simplifications in the assignment. We now work only with the AL5D, and they need to figure out how to construct the Jacobian starting from known transformation matrices for each link.

@Ioana.Ulici I made the changes that we talked over the phone yesterday. I think things are simple enough, but not too simple right now. They still need to figure out how to construct the Jacobian, but they don't have to bother with all the symbolic stuff. Let me know what you think. Feel free to make more commits if you find improvements.