Upgrade libexpat to 2.6.2

[bcoconni]

Upgrade JSBSim local copy of libexpat from 2.4.8 to 2.6.2. This new version includes a number of bugs and security fixes (see libexpat changelog for details).

For the record, JSBSim local copy of libexpat is used by the Python wheels from PyPI in order to support systems where libexpat is not installed (i.e Windows).

However, if libexpat is installed on your computer (including the header/dev files), CMake can compile JSBSim, include its Python module, using the global libexpat library as long as the flag -DSYSTEM_EXPAT=ON is set:

> cmake -DSYSTEM_EXPAT=ON [add-other-flags-as-needed] .

By default SYSTEM_EXPAT is set to OFF even if libexpat is installed on your computer:

jsbsim/CMakeLists.txt

Line 18 in 9bce8d7

option(SYSTEM_EXPAT "Set to ON to build JSBSim using the system libExpat" OFF)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 24.95%. Comparing base (c67311c) to head (e9d79d8).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1080   +/-   ##
=======================================
  Coverage   24.95%   24.95%           
=======================================
  Files         168      168           
  Lines       18072    18072           
=======================================
  Hits         4510     4510           
  Misses      13562    13562           

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