Adds `iterator_diff_` and `iterator_intersect_` functions#2

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doekenorg wants to merge 14 commits intomainfrom

Commits on Aug 14, 2021

Commits on Aug 16, 2021

Added bunch of tests & exceptions. Fixed ColumnIterator.
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Commits on Aug 24, 2021

Code style
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Commits on Aug 26, 2021

Added tests for all exceptions on iterator functions.
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Updated pest
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Fix styling

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Commits on Aug 29, 2021