Cleared Output, missing import, drop inplace, iloc

This commit fixes a few minors in the L3_DataPreparation 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() whenever a plot is generated to avoid
the repr of the Axis object
- the plot_extrapolated_country function had a missing import
of numpy as np
- x = list(range(53)) has been updated as per drop inplace
- iloc is used whenever accessing an entry in the DataFrame.
(Without iloc, the code doesn't work)