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[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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 https://github.com/JSBSim-Team/jsbsim/blob/952ef8df81b8ef596c7f29b3d53f0a0cafd5463e/src/input_output/FGPropertyManager.h#L72
• The reference counting is managed in SGPropertyNode by inheriting SGReferenced https://github.com/JSBSim-Team/jsbsim/blob/952ef8df81b8ef596c7f29b3d53f0a0cafd5463e/src/simgear/props/props.hxx#L755

[agodemar]

Thanks @bcoconni, well done

[seanmcleod]

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

[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 🥳