Option to output only the survey image and not perform analysis

[sowmyakth]

Add command line argument that lets the user to only generate the galsim created survey image for a given input catalog. This would skip over fisher formalism error computations, source and pixel level properties computations. This would be helpful for generating large field images for applications that do not require analysis results, thereby saving the user compute time.

[dkirkby]

This sounds like a useful feature. Can you go ahead and make a PR and ask here if you have any questions about how to implement it.