add_dataframe: attachment of precomputed data (closes #123)

[Enchufa2]

WIP. Some testing needed.

[codecov]

Codecov Report

Merging #140 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #140      +/-   ##
========================================
+ Coverage   99.96%   100%   +0.03%     
========================================
  Files          22     22              
  Lines        2651   2786     +135     
========================================
+ Hits         2650   2786     +136     
+ Misses          1      0       -1

Impacted Files	Coverage Δ
src/simulator.h	100% <100%> (ø)	⬆️
src/process.h	100% <100%> (ø)	⬆️
R/utils.R	100% <100%> (+1.36%)	⬆️
R/wrap-class.R	100% <100%> (ø)	⬆️
R/trajectory-activities.R	100% <100%> (ø)	⬆️
R/simmer-class.R	100% <100%> (ø)	⬆️
src/process.cpp	100% <100%> (ø)	⬆️
src/activity.h	100% <100%> (ø)	⬆️
R/wrap-methods.R	100% <100%> (ø)	⬆️
src/simmer.h	100% <100%> (ø)	⬆️
... and 4 more

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[Enchufa2]

Ready.