Thchang/2355 fix flux density unit

[TienHao]

Description

This PR closes #2355

This fixed the yUnit of flux density in the spectral profiles. The flux density unit is now responsive to the intensity unit.(eg.Jy/beam->Jy, mJy/arcsec^2->mJy, mK->mK).

Checklist

For linked issues (if there are):

• [x] assignee and label added
• [x] GitHub Project estimate added

For the pull request:

• [x] reviewers and assignee added
• [ ] GitHub Project estimate added
• [ ] changelog updated / ~no changelog update needed~
• [x] ~unit test added (for functions with no dependenies)~
• [x] ~API documentation added (for public variables and methods in stores)~

For dependencies:

• [ ] e2e test passing / corresponding fix added / new e2e test created
• [x] ~protobuf version bumped~ / no protobuf version bumped needed
• [x] ~protobuf updated to the latest dev commit~ / no protobuf update needed
• [x] ~corresponding ICD test fix added (BackendService changed)~ / no ICD test fix needed (BackendService unchanged)
• [x] ~user manual prepared (for large new features)~