Implementation of rollback tags

[Enchufa2]

Closes #197. TODO:

• Examples
• Tests
• Bump version and update NEWS

[codecov]

Codecov Report

Merging #287 (6a9b3dd) into master (ad8ea86) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
+ Coverage   99.33%   99.40%   +0.07%     
==========================================
  Files          47       47              
  Lines        2857     2873      +16     
==========================================
+ Hits         2838     2856      +18     
+ Misses         19       17       -2     

Impacted Files	Coverage Δ
R/trajectory-activities.R	98.66% <100.00%> (-0.08%)	⬇️
R/trajectory-methods.R	100.00% <100.00%> (ø)
inst/include/simmer/activity.h	98.14% <100.00%> (+0.14%)	⬆️
inst/include/simmer/activity/rollback.h	100.00% <100.00%> (ø)
src/activity.cpp	100.00% <100.00%> (ø)
inst/include/simmer/process/arrival.h	98.64% <0.00%> (+1.35%)	⬆️

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