Triangle facts feature

Adds angles properties to Triangles class.

Attempts to reduce some of the duplication in calculate_angles by creating a law_of_cosines method and positions array.  I can see how the original three lines are more clear, but it was kind of an exercise in DRY?
id avoid doing this because it doesn't really accomplish anything. you'd be better off just making 3 lines where you call law_of_cosines and pass the 3 sides -- it's extremely straightfoward. that way people who read your code can discern how it's structured almost immediately.