Fixed aliasing issue with relative HPGL export.

[abey79]

Description

Lack of proper rounding could yield aliasing issues with relative export.

To reproduce:

vpype -v begin grid -o 0 0.11mm 1 1200 line 0 0 0 0.11mm end linemerge translate 0.1cm 0 line 0 0 0 13.2cm linemerge layout a4 write -d hp7475a test.hpgl

Both lines should be the same length but the first one is shorter when exported in HPGL without --absolute.

Checklist

• feature/fix implemented
• code formatting ok (black and isort)
• mypy vpype vpype_cli tests 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
  • RTD doc updated and building with no error (make clean && make html in docs/)

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #335 (0159462) into master (ce8026b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #335      +/-   ##
==========================================
+ Coverage   92.31%   92.33%   +0.02%     
==========================================
  Files          50       50              
  Lines        3970     3981      +11     
  Branches      521      521              
==========================================
+ Hits         3665     3676      +11     
  Misses        203      203              
  Partials      102      102              

Impacted Files	Coverage Δ
tests/test_hpgl.py	100.00% <100.00%> (ø)
vpype/io.py	78.23% <100.00%> (+0.07%)	⬆️

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