Fix #206: queueing in multiple resources

[Enchufa2]

See discussion in #206.

[codecov]

Codecov Report

Merging #224 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #224      +/-   ##
==========================================
+ Coverage   99.74%   99.74%   +<.01%     
==========================================
  Files          46       46              
  Lines        2776     2789      +13     
==========================================
+ Hits         2769     2782      +13     
  Misses          7        7

Impacted Files	Coverage Δ
inst/include/simmer/resource/preemptive.h	100% <100%> (ø)	⬆️
inst/include/simmer/process/arrival_impl.h	100% <100%> (ø)	⬆️
inst/include/simmer/process/batched.h	100% <100%> (ø)	⬆️
inst/include/simmer/resource/priority.h	100% <100%> (ø)	⬆️
inst/include/simmer/resource.h	100% <100%> (ø)	⬆️
inst/include/simmer/process/arrival.h	100% <100%> (ø)	⬆️

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