refactored the dvi2whatever code, more appropriately parameterized; added svg as an imagetype, using dvisvgm as the converter
recognize .svg as an extension to generate svg for math images (so far, output to file only)
Updated schema to allow rule attribute on ltx:XMWrap, and to allow labels on ltx:note
added scan handler for ltx:note, to pick up the mark for referencing
updated relocateFootnote to preserve the labels (if any), and to add an id, if needed
in createXMRefs, copy the _box attribute to the XMRef, to keep sensible error reporting
cleanup some comments
set grammar rule for contents of split and similar to allow trailing punctuation
be careful about excess nodes that go away during parsing (eg nested + reduced to n-ary), because these may be referenced by an XMRef