Merge gain-calibration branch

[piyanatk]

We need to start the process of merging the gain-calibration branch to main, so that we can identify parts of the codes that still need updating and refinement, as well as the unit tests that still need to be written.

• @wkhu-astro will tidy up the current code in the gain-calibration branch and submit a PR to merge the branch into main. Tag @piyanatk as a reviewer for the PR.
  • New functionality that will be added should be developed in a new development branch by branching out from gain-calibration. If the gain-calibration PR is merged before the new development branch is ready, it should be submitted as a new PR to be merged into main. Otherwise, the development branch can be merged back to gain-calibration. New commits will automatically show up in the PR.
• @piyanatk will start reviewing the PR to identify what still need to be updated and added. A list of tasks for more code updates and unit tests will be made and delegated.