Move legacy boundary to boundary directory

[ghiggi]

• Closes #xxxx
• Tests added
• Tests passed
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

@@            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	Coverage Δ
unittests	94.09% <100.00%> (-0.17%)	⬇️

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[ghiggi]

@mraspaud @djhoese this is ready for review

[coveralls]

coverage: 93.675% (+0.004%) from 93.671%
when pulling a2dcb46 on ghiggi:refactor-boundary-class
into c39f51e on pytroll:main.

[mraspaud]

LGTM, just one suggestion

[djhoese]

This might be too disruptive, but what about not importing these things in boundary/__init__.py and instead forcing users to import from the legacy module...nevermind that breaks too much backwards compatibility. Looks good.

[ghiggi]

@djhoese @mraspaud you can merge ;)