Review linear_equations (#263)

* Remove figure settings

* Minor updates

* Adjust to PEP8

* Minor updates

* Minor update linear_equations.,d

* Applied review chanegs

* Update long_run_growth.md

Change "GPD" to "GDP"

* Update linear_equations.md

* Minor updates

* Update long_run_growth.md

Change "GPD" to "GDP"

* Minor updates

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Co-authored-by: porntipapoonpolsub <u7618313@anu.edu.au>
Co-authored-by: Sreehari Pulickamadhom Sreedhar <orectique@gmail.com>
Co-authored-by: mmcky <mamckay@gmail.com>

Author: 4 people