Add Intro to Object Oriented program tutorial

[kelle]

This looks like a very useful tutorial written by @bmorris3 which we could consider hosting:
https://github.com/bmorris3/astropy-workshop-bern-2019/blob/master/11-Extras/intro_to_oo.ipynb

[pllim]

Is this really in scope for "astropy tutorials" per se? Do we want to turn this into a computer science course?

[kelle]

• The tutorials site is setup so that there's really no such thing as "too many" tutorials. As long as we have keywords, we can have lots! So I don't think we need to be too picky.

• This particular tutorial is very astro specific and I think thus well suited as an astropy tutorials.