Add std::cin cells to example notebooks

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

After #328 std::cin should work for xeus-cpp, so this adds this to the example notebooks.

Fixes # (issue)

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Added/removed dependencies
• Required documentation updates

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.78%. Comparing base (84b0619) to head (046c493).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #329   +/-   ##
=======================================
  Coverage   81.78%   81.78%           
=======================================
  Files          20       20           
  Lines         950      950           
  Branches       87       87           
=======================================
  Hits          777      777           
  Misses        173      173           

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[anutosh491]

Something looks off here (probably in both cases)

So you've added the cells but not rerun the notebook as such so the configuration stands broken and we can't even see the input box when we do std::cin

[anutosh491]

Also does it really have to be +900-500 change diff to add 2 cells ? Looks a bit too much :\

[anutosh491]

Also not sure what spoiled this

But let's take this opportunity to fix this cell too. We would need to split this into 2 markdown cells so that it is framed correctly

[SylvainCorlay]

Since std::cin interupts the execution of code (preventing a run-all action to complete), maybe we should do it at the very end of the notebook, or just mention it in the markdown cell?

[mcbarton]

@SylvainCorlay @anutosh491 your comments have been addressed. This PR is ready for another review.

[SylvainCorlay]

Thanks.