maint: Update prenorm5.lua and add comments

* dietncl/filter/prenorm5.lua: New function correct_singularity();
Edit function filter.apply (ncl); Edit function turn_binary_ternary().
* dietncl/nclaux.lua: Likewise.
* dietncl/xmlsugar.lua: Likewise.
* tests/test-filter-prenorm5.lua: Likewise.