Add new section to lesson 4

[albhasan]

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Closes #118

This PR:

• adds a new section about removing columns and rows from data frames.
• adds an example and a reference to the functions typeof and na.omit.

These changes addresses the issues raised by @sstevens2, regarding objectives missed by lesson 4.

[github-actions[bot]]

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Rendered Changes

:mag: Inspect the changes: https://github.com/datacarpentry/r-intro-geospatial/compare/md-outputs..md-outputs-PR-149

The following changes were observed in the rendered markdown documents:

 04-data-structures-part2.md | 115 ++++++++++++++++++++++++++++++++++++++++++--
 md5sum.txt                  |   2 +-
 2 files changed, 112 insertions(+), 5 deletions(-)

What does this mean?

If you have source files that require output and figures to be generated (e.g. R Markdown), then it is important to make sure the generated figures and output are reproducible.

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:stopwatch: Updated at 2024-01-11 14:04:50 +0000

[albhasan]

Can I close this PR, @mikemahoney218?

[mikemahoney218]

Sorry, I've been particularly busy recently and haven't had a chance to look this over -- I'll try to check it out this week (unless @likeajumprope gets to it first of course!)