replace galsim object with ndarray image in plot.py showImage

First step toward #14.

remove showArr and elevate ndarray images as the format that showImage
handles instead of galsim objects. Add drawGSObject as a separate
method to extract ndarray images from galsim objects.
update plot.showImage calls in vmapObs

Now that showImage handles arrays, replace showArr calls with
showImage for convOpt=="pixel", and replace showImage calls with
drawGSObject and showImage calls for convOpt=="galsim". (#14)