fix: clarify pandas usage with non-numeric columns

[alee]

pandas now raises errors when computing the mean on a dataframe with non-numeric columns

add a note to the challenge describing the issue and provide a few additional ways of solving it

Thanks to @davidwilby for finding this bug and outlining possible solutions to it in #670

[github-actions]

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The following changes were observed in the rendered markdown documents:

 14-looping-data-sets.md | 20 ++++++++++++++++----
 md5sum.txt              |  2 +-
 2 files changed, 17 insertions(+), 5 deletions(-)

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⏱️ Updated at 2024-03-05 00:49:55 +0000

[alee]

This was also addressed and fixed by @wgriffa in #666 - apologies for missing that and not crediting you with the fix as well!