enable cleaning of internal gdl objects at exit in debug mode

[GillesDuvert]

if gdl is compiled with -DCMAKE_CXX_FLAGS='-DGDL_DO_ATEXIT=1 ...' , i.e, if GDL_DO_ATEXIT is defined, then gdl will clean up all its objects before exit.
This removes a tremendous list of supposed memory leaks as reported by Valgrind or Sanitizers.
To be used in debug mode of course.
Mostly, with the exception of a small list of internal objects that cannot be destroyed before exit (because they share memory with another, destroyed, object) the (still long) list of supposed memory leaks is dues to wxWidgets or other external libraries (probably beacause we do not correctly use their respective cleanup functions).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.83%. Comparing base (e11536c) to head (463cf32).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1762   +/-   ##
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  Coverage   42.83%   42.83%           
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  Files         361      361           
  Lines       97064    97060    -4     
  Branches    19850    19849    -1     
=======================================
- Hits        41577    41576    -1     
+ Misses      55487    55484    -3     

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