JMC: Continuing to flesh out the message passing between models.  Forces & Moments are now summed in EOM and applied.  Need to add a mass properties model so I can tell if this is working properly.
JMC: Several fixes and tweaks.  Integrated states can now be scalars as well as vectors.  liftconfig now handles zero distributions for normal distributions by adding an extremely small value (1E-60) to distributions which contain a zero distribution.  This simplifies input specification without having to customize the numpy.random library and shouldn't have a noticeable effect on results for any units/scales that are typically used.  Finished up the mass props model.  Fixed issue in rocket motor model where mass flow rate was being calculated incorrectly.