[samuelson] Fixing the lecture (#561)

* Fixing the headings to match Quantecon style guidelines and fixing the mathematical equation

* Rectifying the notations in the lecture

This commit rectifies the notations and replaces the greek letters (alpha and beta) with (a and b)

* Update the math equation of the lecture

* Fixing the math alignments after 'Notice that'

* remove global figure size setting

* Rectifying the comments on the samuelson lectures

* Truncating most of the numbers to two values after decimal point

* Hiding the def param_plots() code input which generates the graph

* Replace a and b with unicode characters alpha and beta

* replace unicode characters in text with latex characters

* update math blocks unicode characters with latex notations for consistency

* FIX: matplotlib deprecation warning

* update emphasis from style-guide

* run black (line-length=80) over notebook

* rewrite functions for clarity and remove duplicating code and figures

* minor updates

* revert calculation by hand update

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Co-authored-by: Matt McKay <mmcky@users.noreply.github.com>
Co-authored-by: mmcky <mamckay@gmail.com>
Co-authored-by: Humphrey Yang <u6474961@anu.edu.au>

Author: 3 people