[IntroRemoteDS-pre] Fix and improvements to L3_DataPreparation notebook

[leriomaggio]

Description

This PR fixes a few minors in the L3_DataPreparation.ipynb notebook after coding through the first block of Lecture 3 in Introduction to Remote Data Science pre-release.

This commit addresses Issue OpenMined#380 OpenMined#381 OpenMined#382 Moreover:

• duplicates in raw_data should be applied inplace
• plt.show() has been added whenever a plot is generated to avoid the repr of the Axis object
• the plot_extrapolated_country function had a missing an import numpy as np
• x = list(range(53)) has been updated as per drop in-place
• .iloc is always used whenever accessing an entry in the transposed DataFrame. (Without iloc, the code doesn't work - pandas 1.3.3)

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [ ] I have commented my code following the OpenMined Styleguide
• [ ] I have labeled this PR with the relevant Type labels
• [ ] My changes are covered by tests

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