Error thrown with non-square images

[tjof2]

Code currently fails to deal with non-square images.

[tjof2]

This will be fixed in #8, although it is not clear how best to overcome problems with the quadtree segmentation procedure for noise estimation (which requires a square image at the moment). The PGURE-SVT step doesn't need square images, and will handle them successfully in #8.

The proposed solution for now will be a simple error message, (perhaps best handled in the Python wrapper?), inviting the user to crop/select the area they want to process if they want to do noise estimation. After all, noise estimation may not always be required.