Chapter 14: Integrating Suggestions, Restructure Sections, and Update Exercise (#201)

* Cleaning up sections

* cleaning up sections

* Conditioning

* fix typos

* Apply method

* Update Manipulating DataFrame Section

* Update Subsetting Dataframe

* Revert "Update Subsetting Dataframe"

This reverts commit a9648a5.

* bug fix

* typo fix

* Remove Sony

* change titles and fix typos

* change titles

* fix typos

* fix typos

* reduce repetitions in text

* Add table for variables

* change units

Author: HumphreyYang