MAINT: Update graphviz requirement as removed from build environment

[mmcky]

This PR adds the graphviz python interface package install to the top of lake_model and eigen_II.md

• @HengchengZhang could you also do a grep across all md files for graphviz and make sure all lectures that use it have it installed.

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[HengchengZhang]

Hi @mmcky, I've checked the rest markdown files and both markov_chains_I.md and markov_chains_II.md used graphviz.

In both lectures graphviz was installed by pip but missing the documentation to graphviz

:class: warning If you are running this lecture locally it requires [graphviz](https://www.graphviz.org) to be installed on your computer. Installation instructions for graphviz can be found [here](https://www.graphviz.org/download/) 

[mmcky]

thanks @HengchengZhang can you please update the PR when you are ready. That would be great.

[HengchengZhang]

Updated @mmcky, many thanks.

[mmcky]

thanks @HengchengZhang