Reuse split vertices at same coordinates

[hbruch]

Summary

This PR prevents recreating multiple permanent split vertices at the same coordinate.

Note: this fix is an alternative to #5956, which tries to prevent u-turns by adding costs for u-turns.

Issue

As described in #5955, linking currently creates multiple split vertices for forward and backward edges.
This PR lets VertexLinker keep track of split vertices during graph build and reuses an existing split vertex, if it has the same coordinates as the split coordinates for a different edge.

Closes #5955

Unit tests

A couple of test cases testing the number of created vertices/edges were adapted to reflect the reduced count of edges/nodes.

In addition, this PR provides a test case to illustrate #5955.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.64%. Comparing base (de56d00) to head (66534b5).

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #5985   +/-   ##
==========================================
  Coverage      69.64%   69.64%           
- Complexity     17164    17168    +4     
==========================================
  Files           1943     1943           
  Lines          73795    73808   +13     
  Branches        7552     7554    +2     
==========================================
+ Hits           51394    51406   +12     
+ Misses         19772    19771    -1     
- Partials        2629     2631    +2     

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[leonardehrenfried]

We talked about your PRs today and we would like you to call into one of the Tuesday meetings (where @abyrd tends to be present) to discuss the various approaches.

Personally I'm in favour of treating back and forward edges as a pair so that we can simply reuse the same splitter vertex in the relevant method without having to go through an index. Andrew might have other ideas as well.

[leonardehrenfried]

I will convert this to a draft. You can convert it back to a PR when you start working on it again.