ENH: Generate a visualization of a project's DAG.

[tobiasraabe]

Describe the solution you'd like

Allow to generate visualizations of the project's DAG which is either useful to debug the workflow visually or to flex with the complexity of the project.

The DAG is based on networkx which offers several utilties to plot graph. When we were still using Waf, we used python-graphviz.

We could do the following:

1. Provide a command to render the graph to pdf, png, etc. using a default style.
2. Document in the how to guides how to access the graph when using pytask's programmatic interface.
3. Provide a function which returns a cleaned version of pytask's internal graph such that there is less user interaction with the internals of pytask.

[tobiasraabe]

I, personally, think 2. is a bad idea. 1. increases the complexity of the CLI and I would assume most people would prefer to colorize the graph themselves. Then, 3. is the logical winner.