Reformatting Control Statement Chapter

[SusannaLange]

Reformatted this chapter to be earlier: no data visualization tools used, more intro to boolean arrays, deleted sampling from a DataFrame (moved to Empirical prob chapter). This chapter is titled: Randomness and Control Statements

Edited group, merge, pivot chapter to be three sections instead of two.

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[jesteria]

FYI: The textbook's table of contents in textbook/_toc.yml has to be updated to reflect the file name changes and additions here. These are the warnings I received when building the textbook locally:

textbook-datascience-1$ manage build --all

…

textbook/intro.md:: WARNING: toctree contains reference to nonexisting document '05/placeholder5' [etoc.ref]
textbook/05/1/Intro.ipynb: WARNING: document isn't included in any toctree
textbook/05/2/Control_Statements_conditionals.ipynb: WARNING: document isn't included in any toctree
textbook/05/3/Control_Statements_Iteration.ipynb: WARNING: document isn't included in any toctree
textbook/05/randomness-and-control-statement.md: WARNING: document isn't included in any toctree
textbook/07/1/Merge.ipynb: WARNING: document isn't included in any toctree
textbook/07/2/Groups.ipynb: WARNING: document isn't included in any toctree
textbook/07/3/pivots.ipynb: WARNING: document isn't included in any toctree
textbook/07/merge-group-pivot.md:: WARNING: toctree contains reference to nonexisting document '07/1/Merge_groups' [etoc.ref]
textbook/07/merge-group-pivot.md:: WARNING: toctree contains reference to nonexisting document '07/2/Groups_pivots' [etoc.ref]

[SusannaLange]

Thanks, I just updated the table of contents!

[jesteria]

Cool. I'm still getting an error. (BTW you can probably build the textbook with your changes as well.)

$ manage build --all

textbook/07/merge-group-pivot.md:: WARNING: toctree contains reference to nonexisting document '07/2/pivots' [etoc.ref]
textbook/07/3/pivots.ipynb: WARNING: document isn't included in any toctree

I can just fix this up.

[jesteria]

OK! I'm guessing that change will not offend.

Looks good to me. I can merge if it's ready to go.

[SusannaLange]

Thanks for fixing that for me! It's ready now.