Separate the model geodetic position and orientation into a separate

Separate the model geodetic position and orientation into a separate
class, FGModelPlacement, while FG3DModel retains control of animation.
This way, we can have a single, top-level placement, but multiple
layers of nested models underneath.  To include a nested model, use
something like this in the XML wrapper:

  <model>
   <path>Models/Stuff/my-component.xml</path>
   <offsets>
    <roll-offset-deg>45</roll-offset-deg>
   </offsets>
  </model>