Add an installation test script

[bkmgit]

It may be helpful to add a script that checks whether the packages can be loaded. This would enable learners to know early if their installation is successful. There can be cases where the packages are downloaded, but do not function correctly for the lessons.

[jebyrnes]

This is a great idea - a test script that shows output visualizations you should get! One for terra and one for sf.

[jebyrnes]

(note, I had my students run the demos from helpfiles to check, and they appreciated the immediate feedback)