hardcoded nesting depth of equation groups in XSLT

[xworld21]

Specifically, the handling of equation numbers is unclear if equation groups are nested.

https://github.com/brucemiller/LaTeXML/blob/3875cd64faeb5bdc275d595cb6af772958a51080/lib/LaTeXML/resources/XSLT/LaTeXML-block-xhtml.xsl#L465 seems to assume that there can be at most one nested equation group.

https://github.com/brucemiller/LaTeXML/blob/3875cd64faeb5bdc275d595cb6af772958a51080/lib/LaTeXML/resources/XSLT/LaTeXML-block-xhtml.xsl#L549 checks that no descendant contains tags.

The schema clearly allows for arbitrarily nested equation groups.

I don't know how one ends up with nested equation groups, so this may be a non-issue in practice.