Find circle in a restricted part of the image

[boyska]

Yes, we don't know where circles are. But we may be confident that they are not too far. from the center This issue proposes the creation of an option --restrict-width whose argument should be the percentual (see #7) of image to "keep".
This can help a lot if:

• the camera angle is much wider than the paper, so the calibration paper does not fit it.
• we are confident that the camera is "almost" centered.