Add helio-(bario-) centric time of mid-exposure to OpSim

[hombit]

Feature request

Our simulation pipeline does not account for light-travel time, which may cause problems for fast-evolving and periodic sources.

We can address this in two ways:

1. Add an effect that shifts time from Earth-based time to heliocentric (or barycentric) time. This does not resolve the issue of mid-exposure versus exposure start time.
2. Modify OpSim to provide the heliocentric (or barycentric) time of mid-exposure and use it as the times. It doesn’t seem that LSST provides this in its data releases (DRs), but it is what some users want and is also what ZTF DRs provide as their time column.

Before submitting
Please check the following:

• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.