Create config version of demo13.

[rmjarvis]

This PR is mostly about getting the Roman detector effects into the config layer. There wasn't much back-end code changes required. Indeed the main thing that was required was the implementation of chromatic photon shooting, which I did last fall. Mostly this involved writing new config modules in the galsim.roman sub-module to automatically handle all the standard detector effects.

Some of the new features showcased in demo13 are:

• RomanPSF object type
• RomanSCA image type
• RomanBandpass bandpass type
• The top-level modules field, which we hadn't used yet in other demos in the main demo set (just the des and great3 extra ones)
• Examples of custom modules. I used LogNormal and MixedScene, both from galsim_extra. And I point out that these are both available from that repo.
• Multiple input objects of the same type.
• Multiple rng sequences and the use of rng_num.
• Chromatic photon shooting.

I'm actually a little ambivalent about hosting the Roman config modules in the main galsim repo. I'm concerned that we will want to update these modules on a different cadence than the regular GalSim version updates. However, there is enough pedagogical value in this demo, that I think it is probably still worthwhile as a demo.

But I think fairly soon, the Roman HLS SIT may want to host these separately, so we can update them on a quicker cadence as we work on updating some of the numbers. Especially, we are planning to use different numbers for a number of effects on different SCAs, based on lab measurements of the actual detectors.

[rmjarvis]

BTW, here is a screen shot of the suggested ds9 command that you get from running this demo (either .py or .yaml):

[rmjarvis]

The main reason I decided to add the MixedScene was to get some stars in there, so we could see the spiky Roman PSF. :)

[rmjarvis]

Great. Thanks Rachel!