Improve linemerge algorithm by adding prepending ability

[abey79]

Description

Currently, the greedy algorithm of linemerge would only attempts to "append", which would make it sensitive to line order. With this PR, the algorithm now attempt to "prepend" as well, which improves on the situation.

Consider 3 points A, B, C and the pair of line AB and BC. The greedy algorithm works by picking the first line and merging it with any line whose endings match the end point (in this case B). Here, AB is picked and, since BC shares an endpoint, it is successfully merged.

If the input lines are swapped (BC then AB), the algorithm would first pick BC and look for lines with endpoint close to C. Since there are none, BC is kept as is, and then AB is processed. Since it's now alone, it's is also kept as is and the result is not merged. Now, the algo will also look for lines whose ending is close to B, finds AB, and successfully merges the two lines.

Obviously, the above is done while respecting the no-flip option.

Checklist

• feature/fix implemented
• code formatting ok (black and isort)
• mypy returns no error
• tests added/updated and pytest succeeds
• documentation added/updated
  • command docstring and option/argument help
  • README.md updated (Feature Overview)
  • CHANGELOG.md updated
  • added new command to reference.rst
  • RTD doc updated and building with no error (make clean && make html in docs/)

[codecov]

Codecov Report

Merging #496 (638426c) into master (25454b4) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #496      +/-   ##
==========================================
+ Coverage   94.38%   94.39%   +0.01%     
==========================================
  Files          60       60              
  Lines        5396     5409      +13     
  Branches      786      788       +2     
==========================================
+ Hits         5093     5106      +13     
  Misses        185      185              
  Partials      118      118              

Impacted Files	Coverage Δ
vpype/line_index.py	89.13% <ø> (ø)
tests/test_model.py	100.00% <100.00%> (ø)
vpype/model.py	92.20% <100.00%> (+0.16%)	⬆️

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[sonarqubecloud]

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0 Bugs
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No Coverage information
0.0% Duplication