Add stricter validation for flex areas

[vpaturet]

Summary

Currently the graph builder ignores some types of geometry errors when importing NeTEx flex data.
In particular self-intersecting polygons are imported without any validation error:

This PR applies a stricter validation when parsing polygons so that invalid areas are rejected (i.e. ignored) during graph building.

Issue

No

Unit tests

Added unit test

Documentation

No

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1331f19) 66.87% compared to head (4d5fc5e) 66.87%.
Report is 1 commits behind head on dev-2.x.

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #5457   +/-   ##
==========================================
  Coverage      66.87%   66.87%           
  Complexity     15495    15495           
==========================================
  Files           1800     1800           
  Lines          69801    69803    +2     
  Branches        7353     7354    +1     
==========================================
+ Hits           46676    46678    +2     
- Misses         20665    20666    +1     
+ Partials        2460     2459    -1     

Files	Coverage Δ
...opentripplanner/netex/mapping/FlexStopsMapper.java	86.30% <ø> (-0.19%)	⬇️
...g/opentripplanner/netex/mapping/OpenGisMapper.java	94.73% <100.00%> (+0.98%)	⬆️

... and 2 files with indirect coverage changes

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