Add a "layering" mode to allow users to add objects to already simulated fields.

[cwwalter]

Recently @kadrlica did some dark matter work where he and others simulated a ultra-diffuse galaxy on a pre-existing field by generating the sources he needed with no sky or instrumental background and adding the resulting images to the background image.

With the recent checkpointing code added by @jchiang87 it is always now possible to start accumulating photons on an existing exposure.

As an enhancement, it might be nice in the future to add an official "layering" mode where the user could specify an existing image which already included the sky and other sources and an instance catalog of new sources to simulate.

This way people could take (for example) a subset of imSim DC2 images and then layer/add whatever sorts of sources they wanted to them for specialized studies as Alex did above. This could even be used for small manual sprinkler type studies where a (e.g.) time varying SN was added to a host galaxy in an image. The same base image could be used for each exposure and it would be very fast to add the magnitude varying SN to each one.