fix: GeometryView3D default output directory doesn't work.

[paulgessinger]

In #917 I introduced an output directory to GeometryView3D. I set the default to "." as that usually means the current directory. Due to the way we currently look for file extensions, this didn't actually work. I didn't actually test the default, so that broke the output writing.

This PR works around this by checking if the default value "." is given, and replaces this with the actual $PWD. This is not super robust, and ideally we should revise all of this using boost::filesystem, which I'll try to do in a future PR.

Aside from this, this PR also make the geometry examples correctly pipe through the --output-dir option to the geometry view output.

[codecov]

Codecov Report

Merging #954 (6d579fc) into main (d74d411) will decrease coverage by 0.02%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main     #954      +/-   ##
==========================================
- Coverage   48.62%   48.59%   -0.03%     
==========================================
  Files         334      334              
  Lines       17152    17158       +6     
  Branches     8087     8095       +8     
==========================================
- Hits         8340     8338       -2     
  Misses       3094     3094              
- Partials     5718     5726       +8     

Impacted Files	Coverage Δ
Core/src/Visualization/GeometryView3D.cpp	32.85% <33.33%> (-1.95%)	⬇️

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[asalzburger]

Uncontroversial.