Supports ``weighted average'' of JFM glues.

Supports ``weighted average'' of JFM glues.

- Changed default value of differentjfm to `paverage'.
- Difference between `average' and `paverage' can be seen from

  \documentclass{ltjsarticle}
  \usepackage{lua-visual-debug}
  \begin{document}
  \ltjsetparameter{differentjfm=paverage}．{\Huge ・}\par % default
  ．\inhibitglue\hskip.5\zw\vrule width 0pt{\Huge\hskip.25\zw\inhibitglue・}\par
  \ltjsetparameter{differentjfm=average}．{\Huge ・}
  \end{document}