DOC: Update introduction and preprocessing lessons

[Bradley-Karat]

Added in and expanded upon the current information in the introduction and preprocessing lessons. This includes changes to improve clarity and the logical flow of the lesson. Also changed the first figure of the introduction lesson to something that is a bit more clear (i.e. easier to understand the signal change via gradient direction change).

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[kaitj]

Content-wise, this seems good to me.
Should run this through a linter for formatting / consistency with other lessons

[jhlegarreta]

A few in-line comments.

The old image should be removed from the repository if it is no longer used.

Do not forget to apply them to the Jupyter Notebook.

Also, please squash all commits into a single one in this case.

[jhlegarreta]

@Bradley-Karat would you have time to address the comments or do you prefer to hand it over to somebody else? Thanks.

[jhlegarreta]

@Bradley-Karat would you have time to address the comments or do you prefer to hand it over to somebody else? Thanks.

[Bradley-Karat]

@Bradley-Karat would you have time to address the comments or do you prefer to hand it over to somebody else? Thanks.

Sorry Jon just saw this now. I can go through and address some of these comments.

[jhlegarreta]

Thanks for the effort Bradley. A few notes:

• Can you please add a commit prefix: e.g. DOC to signal that this his a documentation commit? Please see the commit history for guidance on this.
• We have tried to limit the line length to 79 characters so that reading the raw files is easier and for the sake of consistency. Applies to both the Markdown and Jupyter Notebooks. Although the Markdown and Jupyter Notebook rendering does not get affected by the line length, keeping a moderate and consistent number helps a lot when reviewing.

[jhlegarreta]

Also, change the commit subject and message: the preprocessing episode was also changed.

[jhlegarreta]

@Bradley-Karat can you please address the comments made?

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[jhlegarreta]

Please, add a commit prefix (i.e. DOC:) and message body to the commit explaining the changes. This is an excellent post on why this matters.

[jhlegarreta]

@Bradley-Karat Can the comments above be addressed, please?

[jhlegarreta]

Pinging @Bradley-Karat.

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[Bradley-Karat]

@Bradley-Karat Can the comments above be addressed, please?

Sorry Jon just got to this, just made those changes.

[jhlegarreta]

Thanks for addressing the comments @Bradley-Karat.

Usual tests timing out. Merging.