Add get methods and Outputs to compute whole-body momentum from Model

[nickbianco]

It is possible to calculate whole-body linear and angular momentum from a Model by accessing the underlying SimbodyMatterSubsystem:

state = model.initSystem();
matter = model.getMatterSubsystem();
model.realizeVelocity(state);
momentum = matter.calcSystemCentralMomentum(state);

To make these quantities easier to find for users, we should provide methods that make them directly accessible through Model. Adding an Output would also make it possible to directly optimize momentum quantities via MocoOutputGoal.