cluster: better handling of macros

[remimorvan]

For now LaTeX commands are ignored. However some people use macros as synonyms for texts (e.g. $\tw$ and tree-width, or $\wco$ and worst-case optimal join). This leads to some weird behaviour.

For instance if the knowledges \( \sharpP \) and \( \tw \) are defined in different bags and \( \children \) and \( \children(t) \) are undefined, it can happen that \( \children \) is clusterd with the first bag, and \( \children(t) \) with the second one…

Reported by Oliver Irwin.