New exercise: Missing side length in rectangle-composite shapes

In reference to [Trello card #519](https://trello.com/c/SlWTMayL), I've knocked together the second exercise type suggested. I'm not entirely sure of the value of the first (calculating area) as 'area_1' covers it in a way that while is simpler, promotes the process students will undertake anyway.

Grep tells me I'm the only one who is using the createOddShape().labelSides() so my fix here doesn't break anything. I think it is an improvement aesthetically—and I'm relying on it as a cheap way of working out opposite sides.

I'm not sure if this may be better placed merged in with 'perimeter_1'.