Inkscape compatibility mode

This is an incremental release that adds a compatibility mode aimed at Inkscape, that doesn't use relative units.

[0.3.1] - Inkscape compatibility - 2021-10-11

New feature:

• Compatibility mode aimed at Inkscape: allow using px instead of relative
  em units. This allows current versions of Inkscape to load the output
  SVGs. This mode is not on by default.

Fixes, improvements, changes:

• Update documentation to reflect some issues in MathJax math mode delimeter
  choice for hybrid mode.