Refactor/fix HTML math handling.
Use custom exception `utils.math.MathSyntaxError` instead of
abusing `SyntaxError` for LaTeX math syntax errors.
Unify interface of LaTeX -> MathML conversion functions.
Run external MathML converters with `subprocess.run()`.
Revise pre- and post-processing.
Improve error reporting.
Use WARNING instead of ERROR for TeX to MathML conversion errors.
(The source is included in the output as TeX code, so no data is lost.)
Provisional test script for external TeX to MathML converters.
(Placed in a separate directory not to be run by "alltests".
It tests a rather arcane use case, requires optional helpers, and is slow.)
git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@9511 929543f6-e4f2-0310-98a6-ba3bd3dd1d04