Fully working model!!!

Got the model working for both Lab 2 and Lab 3.  One requirement: must
use 3,000 time steps for Lab 2 and 30,000 for Lab 3 in order to see the
expected results.  Should now be able to do assignment for Lab 3, which
will require adding another time differencing scheme, probably either
predictor-corrector, or Runge-Kutta, as mentioned in the book.