Add graphviz pure-Python fallback layout for treeviz

[ammarcsj]

Summary

• Adds a _graphviz_plain_layout fallback that uses the pure-Python graphviz package when pygraphviz (and thus nx_agraph) is not installed
• Removes the "excluded descendants" visual indicator (edge color, legend entry) from tree plots, simplifying the exclusion colour scheme
• Minor fcviz cleanup

Notes

Part of the residual decorrelation PR stack. Stacked on #127.