Parallelize git log entity analysis

[nicolehoess]

Adds parallelization support when analysing git log entities over multiple time windows.

Parallelization relies on the doParallel and doSNOW packages for foreach-loops.

An example implementation is shown using the Spark project configuration file.

[codecov[bot]]

Codecov Report

Merging #234 (733b55f) into master (6ff61e5) will decrease coverage by 0.04%. Report is 1 commits behind head on master. The diff coverage is 0.00%.

:exclamation: Current head 733b55f differs from pull request most recent head 75d9037. Consider uploading reports for the commit 75d9037 to get more accurate results

@@            Coverage Diff            @@
##           master    #234      +/-   ##
=========================================
- Coverage    8.44%   8.41%   -0.04%     
=========================================
  Files          18      18              
  Lines        2356    2365       +9     
=========================================
  Hits          199     199              
- Misses       2157    2166       +9     

Files	Coverage Δ
R/parser.R	8.79% <0.00%> (-0.02%)	:arrow_down:
R/ast.R	0.00% <0.00%> (ø)

[carlosparadis]

Oops! Forgot one more thing @nicolehoess. Could you also rename the notebook so it has as prefix underline? I.e.

gitlog_entity_showcase_parallel.Rmd -> _gitlog_entity_showcase_parallel.Rmd what this will do is, it will prevent pkgdown to try and compile it when generating the itm0 docs page! :)

Thanks again!