HPGL: change velocity to int

[abey79]

Description

• write -f hpgl now only accepts int for -velocity/--vs
• HPGL outputs VS command with int value only

Fixes #255

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 #286 (076db1c) into master (23b4846) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #286   +/-   ##
=======================================
  Coverage   91.72%   91.72%           
=======================================
  Files          50       50           
  Lines        3794     3794           
  Branches      495      495           
=======================================
  Hits         3480     3480           
  Misses        210      210           
  Partials      104      104           

Impacted Files	Coverage Δ
vpype/io.py	75.08% <0.00%> (ø)
vpype_cli/write.py	84.00% <ø> (ø)

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