Fixed issue #434 where forlayer would set _n improperly

[abey79]

Description

forlayer would set _n improperly.

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 #443 (eb4cf03) into master (e5929a1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #443   +/-   ##
=======================================
  Coverage   93.97%   93.98%           
=======================================
  Files          60       60           
  Lines        5298     5301    +3     
  Branches      776      776           
=======================================
+ Hits         4979     4982    +3     
  Misses        204      204           
  Partials      115      115           

Impacted Files	Coverage Δ
tests/test_commands.py	99.03% <100.00%> (+<0.01%)	⬆️
vpype_cli/blocks.py	97.36% <100.00%> (+0.03%)	⬆️

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