add catalog option to make_model_image

[eteq]

This PR does something quite simple: it adds a model_params option to make_model_image. It still requires several things to be ready to be un-drafted:

• Add the option to the subclass' use of make_model_image
• Tests
• Asimple example in the docs?

Before I do any of these though, a key question for @larrybradley : do you like this idea or do you think there's a better way to do this?

The context is that I was talking to a photutils user who wanted to add simulated stars to an image, and they naturally came to the psf machinery and expected to find a way to do it here. A couple of alternatives are available though: 1) just tell people to use datasets.make_model_image. I could modify this in that case to just be a documentation change that shows how to do this starting from a PSF. 2) add something to the PSF base classes instead of expecting them to build a photometry object even though they aren't necessarily planning to do photometry.