Added property to Symbolics that returns the ground reaction force vector expressions.

[moorepants]

@tvdbogert this will at least make the current force expressions more easily accessible. Then it is only a matter of copying the approach for marker tracking but with these values. Of course, it is not the approach of separating the forces from the dynamics model.

You can do this to get expressions for each component:

symbolics.ground_reaction_forces['Right Foot toe'].to_matrix(symbolics.inertial_frame)

[moorepants]

I'll merge this and we can try it in gait-control-id.