Added some updates to the example/notebook.
minor correction to pbe solution
TEST: Lower tolerance on the compute stationary matrices.
Started final push to finish off the Solow model.
Closes issue #77.
Always favor composition over inheritance.
Merge pull request #78 from davidrpugh/hot-fix-ivp

Closes issue #77.
Working on section 2 of the Solow model example notebook.
pulled changes from master.
Effective depreciation rate is now public.
Improved docstrings.
Finished writing section on computing the steady state.