streamlined, simplified validation scripts

[rmandelb]

As discussed in #568 (comment) and subsequent comments, we may want to make a simplified version of the scripts that did the main GalSim validation tests for the GalSim paper, so that we can check that they are still okay when we tag a new version. The latest proposal on that issue was to focus on the 3 key tests that we did, for just the default values of GSParams / interpolants. More discussion and/or work can happen here on this issue.

[rmjarvis]

Propose closing this as an admission that, while a good idea, this is never going to happen.

[rmandelb]

I'd prefer not to give up on this one, though I admit that it being open for 8 years is not promising. It could be a good starter project...

[rmjarvis]

lol ok. :)