Kudo-Defrise phantom/Disc phantom

To simulate the disc phantom

Disc phantom is a widely used phantom in cone-beam ct research, especially in long object problem. It can be used as "P=defrisephantom3d(256);" to output a 256X256X256 phantom. The default phantom is to output phantom with 15 disc. You can use "P=defrisephantom3d(params,256)" to get a phantom different. The use of "params" can be found in the file.