Reduce tolerance when creating cells via contourCoordinate

[svengoldberg]

Quick fix for (the newest part of) issue #933 that causes cells to overlap.
In the long term, this procedure should be recreated in total.

Checklist:

• A test for the new functionality was added.
• All tests run without failure.
• The new code complies with the TiGL style guide.
• New classes have been added to the Python interface.
• API changes were documented properly in tigl.h.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.21%. Comparing base (f6f1e4c) to head (791924b).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1026      +/-   ##
==========================================
+ Coverage   70.19%   70.21%   +0.01%     
==========================================
  Files         301      301              
  Lines       24429    24433       +4     
==========================================
+ Hits        17148    17155       +7     
+ Misses       7281     7278       -3     

Flag	Coverage Δ
unittests	70.21% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/wing/CCPACSWingCell.cpp	78.01% <100.00%> (ø)

... and 3 files with indirect coverage changes