Improve generated C++ header files in the nmodl::ast namespace

[tristan0x]

This contribution provides the following change:

1. Exclude some files from SonarSource analysis
2. Fix a few smells spotted by SonarSource
3. Improve the generated C++ headers in nmodl::ast namespace:

• remove useless copies of std::shared_ptr and std::vector
• use C++17 nested namespace definitions
• the C++ headers have now an acceptable indentation, more readable by developers. I kept the compilation units intact since it is less essential.

You can find attached a diff of the <build>/src/ast directory

ast.patch

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (cda1d1e) 70.18% compared to head (fef71aa) 88.61%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1080       +/-   ##
===========================================
+ Coverage   70.18%   88.61%   +18.42%     
===========================================
  Files         188      169       -19     
  Lines       25627    12474    -13153     
===========================================
- Hits        17987    11054     -6933     
+ Misses       7640     1420     -6220     

Files	Coverage Δ
src/codegen/codegen_cpp_visitor.cpp	85.54% <ø> (-0.01%)	⬇️
src/codegen/codegen_transform_visitor.cpp	100.00% <100.00%> (ø)

... and 23 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[bbpbuildbot]

Logfiles from GitLab pipeline #156739 (:white_check_mark:) have been uploaded here!

Status and direct links:

• ✅ spack_setup
• ✅ build:intel
• ✅ build:nvhpc
• ✅ test:intel
• ✅ test:nvhpc