[price_level] New Lecture on Inflation

[mmcky]

This PR is a new lecture on inflation and price level

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[HumphreyYang]

Hi @jstac,

This PR is ready to merge. I fixed a few minor typos and prevent x-label from overlapping with tick labels.

Could you please kindly review this PR?

Many thanks in advance.

[jstac]

Many thanks @thomassargent30 and also @HumphreyYang for the review. This is a great lecture.

The long term trends in inflation in the advanced countries are striking.

The lecture pip installs xlrd but I think it is never used? @HumphreyYang , could you please check and cut that line if I'm right?

[HumphreyYang]

Hi @jstac, xlrd is used in

# import data
df_fig5 = pd.read_excel('datasets/longprices.xls', sheet_name='all', header=2, index_col=0).iloc[1:]

If we remove it, we will get

ImportError: Missing optional dependency 'xlrd'. Install xlrd >= 1.0.0 for Excel support Use pip or conda to install xlrd.

[jstac]

Roger that @HumphreyYang . Merging.