Extending the functionality of image_undistorter to support the following:

1. We can now set the number of related images in the MVS workspace for patch-match through the new num_related_images option instead of always using the default of 20.
2. We can optionally provide a text files with the image names we want included in the MVS problem through the new input_images option instead of always using all the registered images from the sparse model.
3. We can avoid undistortion and replace it with a simple copy or linking of images when the images are already undistorted and we are not resizing them. The choice of copy or linking is controlled by the new copy_policy option.
4. The undistorter now checks the status of each image undistortion task and only writes out the setup for the images that were successfully undistorted.