setup_reference_from_latex_parts in 16_latex_convert.ipynb cannot replace nested custom commands

[hyunjongkimmath]

For example, if there is a command declaration \newcommand{\field}[1]{\mathbb{#1}} and a command declaration \newcommand{\Z}{\field{Z}}, an invocation $\Z$ is replaced with $\field{Z}$ as opposed to $\mathbb{Z}$.