Main process tells worker when to exit

[alahiff]

This solves #63

[codecov]

Codecov Report

Merging #64 (e8c5262) into main (53e1e2f) will decrease coverage by 0.04%.
The diff coverage is 25.00%.

❗ Current head e8c5262 differs from pull request most recent head 7fecc23. Consider uploading reports for the commit 7fecc23 to get more accurate results

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
- Coverage   37.34%   37.30%   -0.04%     
==========================================
  Files          17       17              
  Lines        4250     4254       +4     
==========================================
  Hits         1587     1587              
- Misses       2663     2667       +4     

Impacted Files	Coverage Δ
simvue/run.py	26.55% <0.00%> (-0.17%)	⬇️
simvue/worker.py	17.64% <25.00%> (-0.15%)	⬇️
simvue/__init__.py	100.00% <100.00%> (ø)

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