Add the alpha command to set the opacity of a layer

[abey79]

Description

There was no way to adjust a layer's opacity without providing the base colour, which may have been set already. This PR introduce the alpha command, which does exactly that. If no colour is defined for the layer, it is set to black.

Also:

• added vpype_cli.FloatRangeType
• fixed typo in vpype_cli.IntRangeType docstring
• fixed misnamed vpype.Color.as_rgb_hex() member name
• added test for alpha
• updated doc and readme
• Fixed error in test_read_command_page_size() on Windows introduced in Improve handling of SVG with missing width and height. #446

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/)

[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 #447 (459c7c6) into master (2d50a1d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #447      +/-   ##
==========================================
+ Coverage   94.32%   94.34%   +0.01%     
==========================================
  Files          60       60              
  Lines        5339     5355      +16     
  Branches      781      783       +2     
==========================================
+ Hits         5036     5052      +16     
  Misses        185      185              
  Partials      118      118              

Impacted Files	Coverage Δ
tests/test_commands.py	99.06% <100.00%> (+0.01%)	⬆️
tests/test_files.py	98.53% <100.00%> (ø)
vpype/io.py	84.57% <100.00%> (ø)
vpype/metadata.py	97.56% <100.00%> (ø)
vpype_cli/metadata.py	92.70% <100.00%> (+0.45%)	⬆️
vpype_cli/types.py	90.07% <100.00%> (+0.21%)	⬆️

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