2d bounding box class for Axis

[martinjrobins]

The Axis class uses std::array<float,4> for its xy limits [xmin,ymin,xmax,ymax]. This should be a separate bounding box class, which allows for:

• adding bounding boxes together to get the minimum box enclosing both: ie. [0,0,1,1] + [0.5,0.5,2,2] = [0,0,2,2]
• translating bounding box by an 2d vector
• scaling bounding box by a scalar amount