Update example des.yaml

[Chihway Chang]

Two problems I've ran into trying to run the example des.yaml.

1. in the des.yaml, it was complaining not getting "image_file_name" and "cat_file_name". I commented out the "images" and "cats" lines and fed it a specific image and catalog... it also didn't like the 'chipnums' line.

2. in psf.py it seems like the argument "copy_image" is obsolete?
   File "/Users/chihwaychang/anaconda3/envs/galsim/lib/python3.6/site-packages/Piff-0.3-py3.6.egg/piff/psf.py", line 196, in
   TypeError: drawStar() got an unexpected keyword argument 'copy_image'

[Mike Jarvis]

Finally done in #118.