WIP noise annotation refactor

[LiangJYu]

This PR builds on @seongsujeong's work from #62. In the early draft stage, f019bae below is the only relevant commit. I'm unsure if changes here should be a PR submitted to #62 or standalone PR pending #62 merging; currently a standalone for higher visibility.

This PR:

• moves noise loading into an independent module.
• only loads azimuth attributes as needed.
• creates utils.py in src/s1reader/utils for common variables/methods. (utils. py may not be the best name?)

To do:

• [ ] test code
• [ ] add unit test

[vbrancat]

@LiangJYu it is unclear to me why we need this refactor. If I am not wrong, @seongsujeong PR has not merged yet. Can we just be done with those changes in his open PR? I think I am missing or I have lost track of something.