Improve pandas code and formatting

[deppen8]

• Add lots of code style formatting where appropriate
• Replace references to isnull() with isna(). These methods are exactly the same for pandas, but isna() is consistent with fillna, dropna, etc.
• Replace a reference to merged_left.genus with merged_left['genus'], the preferred notation style for pandas
• Add more consistent use of {: .output} formatting
• Remove u'string' notation, which is a Python 2.x leftover, to match what learners will see with Python 3.x

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[maxim-belkin]

Thank you, Jacob! Great work! I like the changes you suggest. I'm not 100% sure about the proper formatting for pandas. When we use back ticks we usually talk about code or something we type. Here, pandas is neither but a package name. How about we keep pandas "as was" for now and discuss this change in a separate PR (if you think we should do something to its style)?

[tobyhodges]

See my comment on #467, about pandas vs pandas. As I said there, if you would like to address Maxim's comments here, and adjust the PR to style "pandas" according to this principle, please go ahead and do so, @deppen8. (Note that you will first need to resolve the conflicts that have been introduced.) I will circle back to this one in a few weeks and make the necessary changes before merging otherwise.

[tobyhodges]

Superseded by #541