[PSM] Get the parameter values of the main node when declaring them in the private node.

[JensVanhooydonck]

Description

When creating the PlanningSceneMonitor a private node is created. (This was done here: #266)
This however doesn't use the declared parameters in the main node.

I added a check if the parameters exist and updated the default value of the parameter in the private node to this value.

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (276e661) 50.76% compared to head (6df2e06) 50.78%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2392      +/-   ##
==========================================
+ Coverage   50.76%   50.78%   +0.02%     
==========================================
  Files         386      386              
  Lines       31972    31987      +15     
==========================================
+ Hits        16228    16241      +13     
- Misses      15744    15746       +2     

Files	Coverage Δ
...nning_scene_monitor/src/planning_scene_monitor.cpp	37.74% <77.28%> (+0.47%)	⬆️

... and 2 files with indirect coverage changes

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