#2 Add circle edge detection

[Ed McDonagh]

Need a circle edge detection routine. So many of the phantoms that we need to analyse are circular or have circular objects in, but the included edge detection routines aren't good at finding them.

If there was a Hough Circle Transform (http://docs.opencv.org/2.4/doc/tutorials/imgproc/imgtrans/hough_circle/hough_circle.html) or similar, with parameters of min and max diameter, I think we could do a lot better. Particularly if it would allow us to find circles who's circumference went beyond the image boundry.