Yuhsuan/2015 scripting set channels

[YuHsuan-Hwang]

Description

Closes #2015 by adding new methods setChannel, setStokes, and setStokesByIndex.

• The usage is documented in the codebase. I tried to clarify the usage with two terms:
  Polarization: corresponds to the options in the animator UI.
  
  Stokes: defined as polarization index for standard polarization and polarization value for computed polarization. Used in protobuf messages.
• Refactored code to use the new added methods.
• setStokes is not used in frontend, so I noted “required for carta-python” to prevent deleting it.

Checklist

For linked issues (if there are):

• assignee and label added
• ZenHub issue connection, board status, and estimate updated

For the pull request:

• reviewers and assignee added
• ZenHub estimate, milestone, and release (if needed) added
• e2e test passing / corresponding fix added
• changelog updated / no changelog update needed
• protobuf updated to the latest dev commit / no protobuf update needed
• BackendService unchanged / BackendService changed and corresponding ICD test fix added

[confluence]

This looks sensible to me. I will make a carta-python issue for updating the wrapper code to use this once it is merged.

[kswang1029]

Manual tests are fine as far as I can tell. No regression from e2e tests.

[kswang1029]

@confluence i have merged in this for the python scripting interface.