Remove dependency on boost::property tree ECFLOW-1922

[marcosbento]

Description

As per PR title.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌦️ >> Documentation << 🌦️
https://sites.ecmwf.int/docs/dev-section/ecflow/pull-requests/PR-342

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.61354% with 150 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.40%. Comparing base (52d11c1) to head (c5b4e66).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
Viewer/ecflowUI/src/VConfig.cpp	0.00%	58 Missing ⚠️
Viewer/ecflowUI/src/MenuHandler.cpp	0.00%	26 Missing ⚠️
Viewer/ecflowUI/src/VSettings.cpp	0.00%	18 Missing ⚠️
Viewer/ecflowUI/src/InfoPanelHandler.cpp	0.00%	14 Missing ⚠️
libs/core/src/ecflow/core/PTree.cpp	92.68%	12 Missing ⚠️
Viewer/libViewer/src/Palette.cpp	0.00%	9 Missing ⚠️
Viewer/ecflowUI/src/Highlighter.cpp	0.00%	8 Missing ⚠️
libs/core/test/TestPTree.cpp	99.53%	4 Missing ⚠️
Viewer/ecflowUI/src/VSettings.hpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #342      +/-   ##
===========================================
+ Coverage    50.87%   51.40%   +0.52%     
===========================================
  Files         1239     1242       +3     
  Lines        99249   100303    +1054     
  Branches     14958    15048      +90     
===========================================
+ Hits         50496    51561    +1065     
+ Misses       48753    48742      -11     

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