MAINT: Update graphviz requirement as removed from build environment (#163)

* MAINT: Update graphviz requirement as removed from build environment

* Add warning to graphviz

---------

Co-authored-by: HengCheng <79777246+2789372130@users.noreply.github.com>

Author: mmcky

lectures/eigen_II.md

@@ -20,6 +20,22 @@ kernelspec:
 :depth: 2
 ```
 
+In addition to what's in Anaconda, this lecture will need the following libraries:
+
+```{code-cell} ipython
+---
+tags: [hide-output]
+---
+!pip install graphviz
+```
+
+```{admonition} 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/) 
+```
+
 In this lecture we will begin with the basic properties of nonnegative matrices.
 
 Then we will explore the Perron-Frobenius Theorem and the Neumann Series Lemma, and connect them to applications in Markov chains and networks. 

lectures/lake_model.md

@@ -13,7 +13,22 @@ kernelspec:
 
 # A Lake Model of Employment
 
-\
+In addition to what's in Anaconda, this lecture will need the following libraries:
+
+```{code-cell} ipython
+---
+tags: [hide-output]
+---
+!pip install graphviz
+```
+
+```{admonition} 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/) 
+```
+
 This lecture studies a model of employment and unemployment flows in a large
 population.
 

lectures/markov_chains_I.md

@@ -24,6 +24,13 @@ In addition to what's in Anaconda, this lecture will need the following librarie
 !pip install graphviz
 ```
 
+```{admonition} 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/) 
+```
+
 +++ {"user_expressions": []}
 
 ## Overview

lectures/markov_chains_II.md

@@ -24,6 +24,13 @@ In addition to what's in Anaconda, this lecture will need the following librarie
 !pip install graphviz
 ```
 
+```{admonition} 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/) 
+```
+
 +++ {"user_expressions": []}
 
 ## Overview