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?
method name is not straightforward enough.

example: you could do calc_radians_from_sides and then call convert_to_degrees(radians) within it

using 'calc' and 'convert' tell people exactly what the action of the method is and gives proper context