add a tutorial on making subsystem classes

[zachwaffle4]

the tutorial uses linear slides because all four of my years in ftc used them but a flywheel/shooter mechanism might be more applicable this year; i can add that also later i guess

anyway: I want to add a series of tutorials that involve making classes that aren't OpModes to )help prepare teams for real-life programming scenarios (and of course show them other ways of organizing code).