maint: Add nclaux module; update filter.region to use nclaux

* dietncl/nclaux.lua: New module; auxiliary functions for manipulating
NCL documents.
* dietncl/filter/region.lua: Replace insert_parameter() by nclaux's
insert_descparam().
maint: Syntax cleanup in filter.import

* dietncl/filter/import.lua (resolve_importncl): Likewise.
maint: Add filter.transition

* dietncl/filter/transition.lua: New filter; removes <transition> and
<transitionBase> elements from NCL documents.
* tests/test-filter-transition.lua: New test; checks filter.transition.
* Makefile (TESTS): Updated accordingly.
maint: Comment fix

* dietncl/filter/transition.lua: Likewise.