Have students fit mvgam model before class to trigger additional build steps

[ethanwhite]

Following all the setup instructions for the advanced time-series modeling material doesn't actually fully finish the install. The first time you try to fit a model cmd_stan does some additional source build steps. This is too slow for class and sometimes installs fail entirely at this point. So, we need to have students fit a simple model as part of the install to ensure that the installation is actually complete and works.