add support for variable batch sizes

[Enchufa2]

Enhance batch interface to accept a function as parameter n. This max size is evaluated with the first arrival initiating the batch and stored in the Batched object, which ensures that this limit is not exceeded.

Closes #245.

[codecov]

Codecov Report

Merging #258 (e370cf1) into master (f58ad54) will decrease coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #258      +/-   ##
==========================================
- Coverage   99.64%   99.42%   -0.23%     
==========================================
  Files          47       47              
  Lines        2828     2784      -44     
==========================================
- Hits         2818     2768      -50     
- Misses         10       16       +6     

Impacted Files	Coverage Δ
R/simmer-methods.R	99.54% <ø> (ø)
R/generators.R	100.00% <100.00%> (ø)
R/trajectory-activities.R	98.73% <100.00%> (+0.02%)	⬆️
inst/include/simmer/activity/arrival.h	98.83% <100.00%> (-0.04%)	⬇️
inst/include/simmer/common.h	100.00% <100.00%> (ø)
inst/include/simmer/process/arrival.h	97.14% <100.00%> (-1.38%)	⬇️
inst/include/simmer/process/arrival_impl.h	96.42% <100.00%> (-3.58%)	⬇️
inst/include/simmer/process/batched.h	100.00% <100.00%> (ø)
inst/include/simmer/process/datasrc.h	97.67% <100.00%> (-2.33%)	⬇️
inst/include/simmer/process/generator.h	100.00% <100.00%> (ø)
... and 35 more

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