TODO

* Allow circles to have labels like :C1, not just :C

* Consider whether arcs should be allowed to have labels.
  They do at the moment, just like circles.  But it seems
  unnecessary.

* Consider refactoring such that Circle < Arc.  A circle
  really is a special case of an arc, where the angles are
  0 and 360 degrees!

* Enable the creation of *groups* of objects which can then be copied and
  transformed.

* Consider the area of a circle as a bunch of rectangles, like so

            --             We would want to mark the lengths 2(pi)r
          ------           along the bottom and r up the left side.
        ----------         As the rectangles get thinner, this approaches
      --------------       a triangle.  A = 0.5 * 2(pi)r * 5 = (pi)r^2

  To create the diagram:

    rects = (0..3).map { |i|
      base = 8 - 2*i            # 8, 6, 4, 2
      rectangle :base => base, :height => 1, :centre => ( p(0,0) + i*v(0,1) )
    }

    a, b, c, d = Shape.bounding_box(rects).points

    mark_length :segment => [a,d], :style => :architectural, :padding => 0.5,
                                   :text => 'r'
    mark_length :segment => [a,b], :style => :architectural, :padding => 0.5,
                                   :text => '2(pi)r'

  Some good ideas in there:
   * Creating rectangles with a _centre_
   * Accessing the bounding box for a group of objects
   * Marking lengths

* Access to objects via their labels like Circle[:K].
    - Does this exist already?