Fixed Document.add_to_sources() to also set vp_source

[abey79]

Description

This makes it a more useful API for plug-ins. Also, ValueError is no longer silenced.

Fixes #412

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 #431 (0fbb7e7) into master (b3907ed) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
+ Coverage   93.93%   93.96%   +0.03%     
==========================================
  Files          60       60              
  Lines        5276     5288      +12     
  Branches      774      775       +1     
==========================================
+ Hits         4956     4969      +13     
  Misses        204      204              
+ Partials      116      115       -1     

Impacted Files	Coverage Δ
vpype/io.py	84.61% <ø> (-0.08%)	⬇️
tests/test_model.py	100.00% <100.00%> (ø)
vpype/model.py	92.04% <100.00%> (+0.26%)	⬆️
vpype_cli/read.py	96.07% <100.00%> (+0.16%)	⬆️

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication