Dev eg

[emilygrabowski]

This PR:

• Revises the preprocessing notebook to use the penguins data set
• Revises the classification notebook to include challenge questions
• Adds some general edits for clarity

[pssachdeva]

Thanks for the PR. Could you make the following changes:

• Could you number the challenges, and add a title to each one (e.g. Challenge [NUMBER]: [CHALLENGE TITLE]).
• I'm also playing around with adding a horizontal line (--- in Markdown) at the top and bottom of the Challenge cell, just to demarcate it from the rest of the text. Can you add these just so it looks consistent?
• I have a pip install cell in the first notebook, so I think you can remove the sentence about installing sklearn in the classification notebook.
• Could you add an equation in the logistic regression section? I think it'd be good to have it there just so people see exactly what the beta values are. I think I have some Markdown text explaining this in detail in the Python Text Analysis workshop (see the bottom of this notebook) that you can feel free to copy.
• Can you rename the Multiclass classification section "Decision Trees"? Logistic regression can be used for multiple classes, so change the introduction paragraph to just highlight that. Adding a picture in this section of a decision tree would also be good.
• There is an extra_code.ipynb file, but nothing in it. If there is supposed to be bonus material, could you place them in the corresponding notebook (e.g., in regression, we have KNN regression as a bonus). Could you remove that notebook?
• This could be in its own PR, but could you add separate solutions files for each of the preprocessing and classification notebooks? Then, remove the solutions.ipynb file that's already there.

General style changes:

• Can you make each comment begin with a capital letter, and put a space between the hashtag and start of text?
• Remove spaces around equal signs when passing arguments into a function or object
• Using f-strings in print statements rather than .format()
• For each challenge, have a cell with top line # YOUR CODE HERE

Once you've made these changes, feel free to merge.

[emilygrabowski]

This PR:

• Addresses content and style for Notebooks 3/4
• Add solutions notebooks for 3/4