Simplify gitignore

[ischoegl]

Checklist

• There is a clear use-case for this code change
• The commit message has a short title & references relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• The pull request is ready for review

If applicable, fill in the issue number this pull request is fixing

• In Improve error message for missing phases key #880 renamed executables were excluded in the main .gitignore file (rather than test_problems/.gitignore)
• Hidden .gitinore files in subfolders are hard to locate / easy to overlook
• Merge test_problems/.gititnore (and others) into .gitignore
• Use include overrides in addition to exclude directives to declutter

Changes result in a reduction of gitignore instructions by half

[codecov]

Codecov Report

Merging #888 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #888   +/-   ##
=======================================
  Coverage   71.11%   71.11%           
=======================================
  Files         376      376           
  Lines       46201    46201           
=======================================
  Hits        32858    32858           
  Misses      13343    13343           

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