Make the Python binding of FGPropertyNode compatible with SGSharedPtr ref counting

[bcoconni]

This PR modifies the Python binding of FGPropertyNode so that the reference counting of the SGPropertyNode instance that it points to is updated.

This is to avoid that the C++ instance of FGPropertyNode/SGPropertyNode that the Python object relies on is being deleted by another event. Such a scenario would result in a dangling pointer and eventually to a segfault of the Python interpreter.

In other words, this PR is meant to avoid the scenario below from occurring.

fdm = jsbsim.FGFDMExec()
pm = fdm.get_property_manager()
node = pm.get_node('some/node')

# [...] A lot of code

del fdm

# [...] Some code until the Python garbage collector actually deletes fdm

node.get_double_value()  # BOOOMM !!!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (952ef8d) 24.87% compared to head (d30e99f) 24.87%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1000   +/-   ##
=======================================
  Coverage   24.87%   24.87%           
=======================================
  Files         168      168           
  Lines       18908    18908           
=======================================
  Hits         4704     4704           
  Misses      14204    14204           

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[bcoconni]

For those wondering what the difference is between FGPropertyNode and SGPropertyNode:

• FGPropertyNode inherits from SGPropertyNode
  

  jsbsim/src/input_output/FGPropertyManager.h

  Line 72 in 952ef8d

  class JSBSIM_API FGPropertyNode : public SGPropertyNode

• The reference counting is managed in SGPropertyNode by inheriting SGReferenced
  

  jsbsim/src/simgear/props/props.hxx

  Line 755 in 952ef8d

  class JSBSIM_API SGPropertyNode : public SGReferenced

[agodemar]

Thanks @bcoconni, well done

[seanmcleod]

@bcoconni @agodemar quite a milestone, the 1000th pull request for the JSBSim repo at Github! 😉

[bcoconni]

It's actually the 298th-ish PR: the number 1000 is actually a counter that accumulates over all the items in the repo (PR, issues and discussions).

That's quite a feat nonetheless. Congrats to the community 🥳