Implements massless position controlled control surfaces

Works on #107 by implementing position controlled control surfaces via linearizing around u = 0 to create a B matrix for the control surfaces.  Includes a unit test and, while changing a substantial amount under the hood, does not modify the existing RigidBodyWing interface and testWing should still pass.

I did this by implementing RigidBodySubWing classes that dealt with the actual computation and a RigidBodyWing (which can contain multiple subwings) sums up the forces created by its subwings.  The old interface is implemented by creating a single subwing and having calls basically pass right through RigidBodyWing to its instance of the RigidBodySubWing.  @blandry should take a look.
presumably, you need to add urdf documentation for the visual and contact geometry fields on the wing class..?