Move legacy boundary to boundary directory

[ghiggi]

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

In this PR, I just moved the legacy boundary.py to the new boundary directory named as legacy_boundary.py

[codecov[bot]]

Codecov Report

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

Comparison is base (ba09e7d) 94.25% compared to head (a2dcb46) 94.09%. Report is 57 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #565 +/- ## ========================================== - Coverage 94.25% 94.09% -0.17% ========================================== Files 82 85 +3 Lines 13077 13235 +158 ========================================== + Hits 12326 12453 +127 - Misses 751 782 +31 ``` | [Flag](https://app.codecov.io/gh/pytroll/pyresample/pull/565/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/565/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `94.09% <100.00%> (-0.17%)` | :arrow_down: | 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]

@mraspaud @djhoese this is ready for review

[coveralls]

coverage: 93.675% (+0.004%) from 93.671% when pulling a2dcb4684c7a542fb164118a03c57b9ea682c296 on ghiggi:refactor-boundary-class into c39f51efe0d769e4b9b1d8b930c4dad988891757 on pytroll:main.

[ghiggi]

@djhoese @mraspaud you can merge ;)