Add `BoundarySides` class

[ghiggi]

• [ ] Closes #xxxx
• [x] Tests added
• [x] Tests passed
• [x] Passes git diff origin/main **/*py | flake8 --diff
• [ ] Fully documented

This PR add the BoundarySides class to facilitate the manipulation of boundary sides.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (c240c04) 94.09% compared to head (47e2004) 94.12%. Report is 10 commits behind head on main.

:exclamation: Current head 47e2004 differs from pull request most recent head a2e9fb1. Consider uploading reports for the commit a2e9fb1 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #568 +/- ## ========================================== + Coverage 94.09% 94.12% +0.03% ========================================== Files 85 87 +2 Lines 13235 13312 +77 ========================================== + Hits 12453 12530 +77 Misses 782 782 ``` | [Flag](https://app.codecov.io/gh/pytroll/pyresample/pull/568/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pytroll/pyresample/pull/568/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `94.12% <100.00%> (+0.03%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ghiggi]

@djhoese I think this is also ready to be merged. I will restart working on the other PR on Monday ;) Have a nice week-end

[coveralls]

coverage: 93.711% (+0.04%) from 93.675% when pulling 47e2004a1517a92f92eaf34a9323306351713262 on ghiggi:add-boundary-sides-class into c240c0492ac5fd35add30d3c9fbcf3c45aba36ba on pytroll:main.